From the many, many flights I have taken, I think, generally, the seats are large enough to accommodate an above-average size man. I'm 6' 1", about 200 lbs, just a bit above average, maybe. I don't have tons of room in coach, but I'm not really wedged in, either. Now, if you are morbidly obese, then you have a problem. It is not the airlines problem. Unless, that is, they ignore the problem, then I have to be lain all over by the obese guy in the seat next to me (and also, about 1/3 into my seat).
This happened to me on a 3 hour flight where I was in a two-seat row next to a man who had to be at least 400 pounds. The divider/armrest between the seats literally could not be lowered. The only way I could sit was with my legs at about a 40 angle toward the center aisle, and my torso at about a 30 degree angle away from this guy. I could not sit this way for long, of course, and ended up standing throughout the flight, only sitting for take-off and just before touch-down. Needless to say, I was not a happy camper and I complained to the flight attendant. She could only say, "Yeah, we should have caught that." Ya think?