My wife tonight argued that cup size is a ratio of rib cage size to actual boob flesh. As in, a 34 D has less actual breast than a 38 D.
That doesn’t sound right, but I’ve been wrong before.
She’s right. The cup in a 34 D is smaller than the cup in a 38 D. Go down to the lingerie department, open up a few bra boxes, and examine the cups. Then explain to the nice security guard that you’re doing this in the name of Research. 
Wow, learn something new every day. Leave it to a women’s clothes to have sizes with no objective measurement. 
I have a 11 inch penis…for a 5 foot tall man.
Who said no objective measurement? The number is inches around the rib cage, and the letter is how many inches more than that number the breast measurement is (A=1", B=2", etc.), so a 34D is 34 inches of band and 38 inches around the breasts. A 38D is 38 inches of band and 42 inches around the breasts.
Like here.
Actually, bras are the one piece of women’s clothing that does indeed have an objective measurement.
Bra cups are sized by how many inches bigger your full bust measurement is from your underbust measurement. 1"= A, 2"= B, 3"= C, etc. So the cup volume is, essentially, the difference of the area of Circle A and Circle B. (Assume a round body for ease of calculation.) Bigger circles gives you a bigger difference in area for the same difference in circumference.
It is right. I’m a 38DD and can eyeball my size by “will the cups fit like a hat/skullcap on my head?” as a factor. When I was a 36DD, this did not work anywhere near as well. I have never been a 34 anything, let alone a 34DD (big ribcage), but they’re definitely a lot smaller proportionately than the higher ribcage sizes. Another way this is true is the concept of “sister sizing”: if you can’t find a bra in your “true” size (the one that fits you best), you can sometimes make do with bras that are a cup size up and a band size down or vice versa. (Ex: If I can’t find a 38DD, I can try a 40D or a 36DDD instead. I’ve done this before, and one of my favorite bras happens to be a 36DDD.)
This concept explains why I have the volume of breast to fill a 32A cup, but the band is ginormous on me and the underwires really hurt - while my 28Cs fit like a dream. They hold the same amount of boob, but have a much shorter band and underwires that are the right shape.
28C, 30B, 32A, 34AA all hold roughly the same amount of boob (with allowances for brand, style, and manufacturing differences), but are designed for very different sizes and shapes.
Same for every bra size. It makes it really, really easy to wear bras that don’t actually fit so well for your entire life - women think that if their boobs can be contained by the cups, the whole bra ‘fits’. But the sizing of the band and spacing/shape of the cups is of utmost importance in support and comfort. Most women are wearing bands that are too large, in part because there are so few options in America if you have a small ribcage or large breasts for your frame. Many women have never even heard of size 30 bands (outside of 30A training bras), much less 28s.
I’m a believer also in wearing a band size that is quite close to the actual measurement of your ribcage… many bra fitters agree. Some don’t.
If your bras are uncomfortable or not supportive in any way, the first thing I tell everyone is to try on the size one band down, and 1-2 cup sizes up. It’s almost magical what a difference it can make.
For those of us on the other end of the spectrum (36DDD here) sometimes doing the reverse of that (getting a larger band with a smaller cup) can work wonders, too. Don’t worry that you are going down from a D to a C (or in my case from a G to a DDD), your boobs did not get smaller.
http://www.straightdope.com/columns/read/77/am-i-imagining-or-are-womens-breasts-getting-bigger
Cecil on bra sizes
I’m sorry, but how are grown women wearing training bras? I get that grown women might still have small breasts, but their chests & rib cages & such are bound to be larger than that of a pubescent girl. 
Do you not understand that women, even as adults, can come in different shapes and forms? “Training bra” is just a pejoritive term for a bra with small dimensions.
Since I (and another poster in this thread) wore a 32B when I was 10, and rhubarbarin wears a 28C (which is narrower around the ribcage, with slightly less cup volume), this is obviously incorrect. Her adult boobular dimensions are more petite than I was in the 5th grade.
I stand corrected on both points. Thank you.
Also, they are not actually training anyone’s breasts, but remain silent throughout the growth process.
In HeyHomie’s defense, I do own a lot of girl’s size 12 clothing. It’s very unusual for a woman in her mid-20s to be as small as I am. But I’ve always been much smaller than everyone else my age.