You get the extra muscle from hauling your fat ass around. Once you get too fat to move, the muscle isn’t needed, and so your body sheds it because it’s metabolically expensive.
Also, yes, fat guy strength is a well-known phenomenon among gym rats. Most big fat dudes don’t have to work too hard to get up to a bodyweight deadlift.
While the study I quoted found pretty clearly that
(SM = skeletal muscle) Figure 4.
it surely varies greatly between fit fat and the unfit fat. If someone is actually regularly hauling that mass out and about, up and down stairs, let alone actually doing things like dancing or jogging or even weights, (are fit, like ZipperJJ) then they likely have more muscle mass. To the degree that they are fat because they are not doing those things and to the degree that being fat prevents them from doing those things (are unfit) they likely have less muscle mass.
Yeah, or work too hard to get up to a big bench-press either; considering that the bar only has to travel about an inch and a half down to the giant belly waiting for it to be bounced against.
“Obese” people are hugely variable in their activity levels. If you are reasonably active you will have some muscle mass as a result of hoisting your weight around, if you are largely sedentary then not so much and even moving is a struggle.
As others have said if you lose the weight mainly through dieting, and do not exercise intensively and eat enough protein in your diet you will rapidly lose that muscle support mass while losing fat.
The other issue is that muscle mass due to carrying/supporting a fat physique is usually not all that well toned and can do short bursts of strength, but quickly tires.
I speak from experience in this.
You bench press to your belly not your chest?
No. But I’m not a fat guy trying to “lift big” at the expense of lifting correctly, either.
It seems as if what you are asking is this: If a large man suddenly lost all his fat but not any muscle mass, would he be a fairly strong guy?
Let us assume this previously large man was mostly fat–i.e., not a muscle builder–but could get up and walk around reasonably well–i.e., was not bed-bound.
In that scenario, we know that his legs could at least support his prior total weight. If he could squat and get up before, and now he suddenly weighs only 180 pounds, then he is now strong enough to squat and lift 220 extra pounds on his shoulders. And so on.
Various other muscles might be pretty weak. He might not, for instance, be able to actually hold that 220 pounds very well. He might not be able to do a pushup.
In actual circumstance, as mentioned above, weight loss reduces muscle mass along with fat.
Muscle strength varies quite a bit in the obese. You will notice that quite a few active fat people–men, especially–have fabulous calf muscles. But many fat people tend to use their skeletal systems for support and their muscles serve mostly to keep them balanced. Many are barely able to do much more than shuffle along til they get to the nearest Walmart motorized cart. If they go down, they stay down.
Here’s a classic Straight Dope column dealing with this question (see second paragraph of the response).
I’m a somewhat active fat guy, weighing in at 220, about 5’8". I walk everywhere (no car) - so that’s at least 30 minutes a day, hit the gym every now and then, take the stairs at work, and will do light stuff like throwing a football around, playing frisbee, or whatever. My calves are ridiculously big and buff, if you saw me from the knees down, you might think I’m an Olympian.
I managed to make it to the gym this morning for 1 hour, 3 1/4 mile walk on the treadmill on a 5 degree incline. That’s a normal workout for me, and I have to think that hauling my big ole gut around doing stuff like that is why my calves are so shredded. Now if I could just get the rest of my body to look like that…
I’m ex-obese and I have fantastic calfs from a lifetime of being fat. I don’t bother with lower body when I am lifting… 
Same here (well not ex-obese). I said earlier that my thighs and glutes are huge but my calves are well-defined and muscular as well. I’ve never had a “cankle” (I’m a woman…I think women are more prone to cankles?) That’s why I decided to get my tattoo up the inside of my ankle/calf - it’s one of the few places on me that never changes shape or size.
Many die young, actually.
However, during their career, many are both strong and have a lot of fat on their bodies. It makes them incredibly powerful. I think they have a very tough diet and workout system to make them stay at that level.
It can’t be too good for the heart, though.
Indeed. There can also be an advantage in lifts like squats and cleans, where the belly pushes against the thighs and adds leverage.
There is also a phenomenon called “intramuscular leverage”, where (IIRC) a muscle with higher fat levels can develop higher force levels because the muscle fibers can squeeze against the fat and contract harder. I only read about it once, in a powerlifting manual, and I can’t find an online cite. So take that FWIW.
Regards,
Shodan
I just wanted to take a moment to say thank you for all the great replies! 
I’ve enjoyed reading all of them so far.
Also, congrats to all those that have won the battle of the bulge! No easy task, I know.