Apple-shape here. I gain weight around my middle before I gain it anywhere else. I’m 5’5 and small-boned, but even when I reached the point where I could have cut someone with my clavicles (about a hundred and twenty pounds) I still had belly bulge.
Cup size is related to breast size, the number following is the ribcage measurement. The way one decides what size bra to buy, is by measuring, first the ribcage, then the bust line.
Here is a bra size calculator if you follow the link at the bottom, it explains how band/cup sizes are determined. It’s different is different countries.
This isn’t true before menopause. Women with normal BMIs and big breasts have mammary tissue, as one ages, that becomes replaced with fat.
BTW, did you know that mammary glands are just modified sweat glands? 
Yep, it’s not true that boobs are ‘fat’. The amount and density of the gladular/connective tissue in boobs varies a lot from woman to woman. Women who have a higher bodyfat percentage are more likely to have more fat in their breasts of course. But they can also have a much higher amount of dense breast tissue than someone else with boobs the same size. The more actual breast tissue as opposed to fat you have in your breasts, the higher your risk of breast cancer, and the more difficult it is to read your mammogram (you’re also much more likely to have a false positive when you get a mammogram). As you age and after menopause, the hormonally-governed glandular and connective tissue wastes away, so your boobs get much fattier (or just saggier if no fat moves in to fill out the space).
My own breasts are small (although I wear a B or C cup, it’s on a 28 or 30 band - I look ‘flat-chested’) but they are almost 100% dense breast tissue with just the tiniest amount of fat under the skin. I really don’t hold any fat to speak of on my torso, boobs are no exception.
Yes, modified sweat glands. Made of fibrous tissues and fat.
Breasts are made of 15-20ish milk glands that are connected to milk ducts and meet at the nipple. Most of the rest of the breast is fatty tissue and fibrous connective tissue.
Well, I like 'em sweaty.
IME… Maybe.
I don’t think it’s even remotely as simple as looking at chest size and drawing a conclusion. I’ve known very thick women with very small breasts, and very slender women with natural breasts that were almost freakishly large.
One of the best rants ever on the subject…Rosie O’Donnell in the movie “Beautiful Girls.”
I’m another one who can gain or lose an astounding amount of weight without changing much in the boobage. A 50 pound weight loss saw me going down 2 inches in band size, but stayed the same in the cup.
No, not really. The bust measurement (across the nipples) will be smaller if you’re a 32C, but the difference between the ribcage and bust measurements will be 7 inches in both cases. The ribcage is smaller, the boobage still takes up 7 inches of circumference.
If those sizes are correctly fitted, then a 32C woman will have a 28 inch ribcage and 35 inch bustline. A 38C woman will have a 34 inch ribcage and 41 inch bustline. 7 inches of boobage either way, only on a smaller ribcage at 32C. HerRoom: Women's Lingerie, Bras, Underwear, Panties, & More
All that being said, yes, I think the OP is correct, for the estrogen related weight gain reasons **TruCelt **brings up. Higher fat (in the boobs and elsewhere) leads to insulin resistance, which leads the body to put on more fat, which leads to greater insulin resistance, which leads the body to put on more fat… While it’s possible to be slim with big boobs, it’s hormonally unlikely to stay that way without effort past your 20s.
I agree with your main point - the cup size is the proportion of your breasts to your body. But the cups size roughly corresponding to the number of inches difference between underbust and overbust measurement is only a rough estimate of your size range. A woman who has a 28" ribcage and 35" bustline can have a tiny body frame and full boobs, or have a chest that broadens suddenly right above the underbust and well-developed lats like me, so there is much less breast tissue in that 35" overbust measurement.
The extremely limited bra size range in most of the world complicates the matter as well. There are women with ribcages 25-32" who are wearing 32 bands as that’s the smallest size available, they don’t all have consistent proportions corresponding to the size bra they wear most often.
Way to turn me gay.
Great, you know they’re all accompanied by their gay BFF? 
True, most women don’t wear the right size bra, which is why I wrote, “if those sizes are correctly fitted.” The band size should be measured right under the breast tissue, so well developed lats should be figured in the measuring process.
The cup size really is a simple add on formula until you get past a D cup or a band size over 40. Band size = underbust+4 (if even) or underbust+5 (if odd). Cup size = bust (over the nipple)-band size. 1=A, 2=B, 3=C, 4=D
For larger women, it is a crapshoot, as manufacturers use all sorts of different measurement systems. I’ve had 42DDD bras that fit well and 38I bras that fit well, as fitted by professionals. There’s just too much manufacturer variance to rely on numbers when you’re plus sized.
I just don’t agree with the ‘add 5’ rule. First of all, it has never worked for me and doesn’t for most very small women. Underbust 26", +4=30 (which I had never seen in stores), okay then +5=31, round up to 32 as that’s what’s available. However my overbust is 31. -1=AA? But I fill out plenty of 32As! They just ride up all the time, and the underwires dig into the outside edge of my boob painfully.
It puts almost everyone in a band far too loose to provide most of the support, if it is needed. When the band isn’t tight the weight of the breasts pull on the shoulder straps, which can cause serious issues. This depends somewhat on manufacturer, but 32s are enormous on me. There’s also the rule that you shouldn’t be able to pull your bra band more than a little bit away from your back… I consider this a much more reliable barometer of whether a bra works for you. I could fit another ribcage my size inside most of the 32 bands I used to wear.
Also I was mostly referring to both my teres major and minor muscles and the upper part of my latissiumus dorsi when I said ‘lats’. They are largest at the same level my boobs sit, so increase my overbust measurement. My band fits under them.
Try here: Types of Bras - Bra Space They’ve got stuff in 30 bands. You can also try stores that cater to younger women, y’know, the really hip places in the mall playing terrible “music” at inhuman levels.
Okay, but then it won’t fit. Not because the measuring system is wrong, but because you’re not using it because you can’t find the size you really need.
I think I understand now what you mean about your back muscles, though. My best friend growing up was a state champion butterfly swimmer, and her shoulders and upper back were…problematic when it came to fitting dresses and such. Basically, you’re saying that the increase in your bust compared to your underbust doesn’t just come from your boobage, but also from your back. Interesting…yes, that would make cup sizing a bitch.
And makes online ordering a bitch. When my bra model changes (or I gain or loose more than 10 lbs), I spend an hour at Nordstrom with the 'bra lady" trying on bras. I measure out as a 32 DDD or something (I have a 30 F, which is what I measure to, but the band is too tight for comfort), and after trying on 30 bras in the same size - only two models will fit.
I cannot imagine buying a bra online.
I should go and get fitted again. I’m one of those women who evidently changes a lot with weight gain - I used to be one of those high metabolism size 2s, and I had to hunt up and down for the one kind of bra that fit me - from Playtex, called I shit you not “Thank Goodness it Fits”, size “nearly A”.
Hell, I didn’t know how good I had it. I gained 40 or 50 pounds and was wearing a bra way too small (B) when I had the bra lady spend some time with me - now I’m either a C or a D depending on the brand and it is an enormous pain in the ass. They’re always in the way, it affects my golf swing, people stare at them when I lean over to do my damned job.
I used to hate shopping for certain things like fitted dresses because they were all made for bigger boobs than I had. Now I can’t cram all my boobage into stuff and I’ve popped buttons off once or twice. Arrrrgh!
I was once told by a coworker with big boobs and a big ass, but otherwise pretty slim, that if you have big natural boobs, you also need to have a big caboose to counterbalance it, and any woman I see with big boobs and a tiny ass is fake.
Any truth to that?
No, I have a girlfriend I’ve known since she was 15. She is one of those women who developed an impressive rack - but has no ass. She calls herself “the incredible assless woman.” I’ve known her forever, I’d know if her boobs were fake. And since she is the “I don’t dye my hair because that would be putting CHEMICALS on my body” and “I use an organic face cream” type of hippie - and always has been, the idea that she’d pump up her breasts with silicone is not reasonable - even if she’d slipped away and had them done.
Nothing is universally true, of course, but generally speaking, yes, for the same estrogen related reasons **TruCelt **brought up upthread. More fat=more estrogen=more fat in other places on the body. But of course, there are always exceptions.
Many women in my family are stacked on top, but lack in the back (thanks, mom!). OTOH, they have big hips (hourglass).
My sister (different mom) got stacked on top and back, but relatively speaking, she is not much of an hourglass.
That simple difference makes it easy for me to get my sis hand me downs. What doesn’t fill in the back is fill in the sides. And the top is always full.