I'd say a combination of not enough food and consistency and progress on strength. I think for somebody wanting to go up in size, they need a good long break from competing. That doesn't correlate into turning into a fatass though, and I think a lot of people have a hard time distinguishing between the two. They either get way too fat and sloppy, or don't eat enough and look the same year after year. I'm not talking about looking softer from a little added fat, distended gut, and water retention either. I'd say, and I believe even John meadows has stated (correct this if I'm wrong), that if you can still see an outline of your abs, then keep going. Once it is gone, take a small cut and get it back under control. I see women eating more than some guys sometimes, lol. Some guys use a certain BF% as a line in the sand to not go over. IMO< I don't think somebody taking a 2-4 month off-season is going to gain much from the prior year or contest, if they are already pretty maxed out, that may happen with a newb, but once you are at your peak, it takes a lot more time to get more muscle. At some point you need to be on a caloric excess to continue gaining weight, along with some time. Where that caloric excess land with each person will probably vary. IMO, people would probably do better with a gradual increase over a longer period. Somebody cutting on 2500kcals might not want to immediately head up to 5-6,000 kcals....but again, this is going to vary person to person.
Guys like Meadows and BBOY, have aton of muscle, they have pretty much all they need (BBOY, correct me brother if I am wrong), so they are in a little different category. I don't think people realize how much muscle these guys carry.
I have to agree with both BigA and DoggCrapp from prior posts at ProM...
With DC (and others), if you have top genetics, you will know in a few months of training when you start passing others up who have been doing the same thing you are for years.
BigA mentioned that anybody can get bigger, but the genetic elite will have better appearance, roundness, muscle fullness, muscle insertions....you know, the look of a pro or somebody with good/great genetics. I agree with this too.
I think almost everybody can get pretty big, maybe a few with real shit for genetics can't, but for the most part everybody can get bigger or improve. You are not going to take a big guy like Hulk Hogan and make him into a top IFBB pro on a stage, he has the size, but still not the shape. He has genetics to get big, even at like what nearly 7' in height?
I do believe there are some guys, take Lyle McDonald for example, who just are never going to be real big. Talk about somebody getting the short end of the stick on genetics, it's him. Now, imo, this guy is a fukking genious, and i'd follow any advice he threw my way, I gobble up his postings, and writings, guy is top notch. However, even with all his knowledge and even if he had say Farrah or Chris coaching him (not that I think he would need it)....he's probably not going to go too far in the size department. IMO, some people need to keep things real and in perspective. And in this case, Lyle actually can get pretty lean, albeit, a very small guy, and with different goals, but he found what works and does best for him.