The pattern is to me pretty obvious at this point, having worked out at several different gyms at completely different parts of the country. The guys who are really big work out a lot less than the rest of us. They take long breaks between sets and are often seen just talking with people on the bench. I could think of a bunch of reasons why this seems to be the case:

(a) They don't feel like putting on much more weight than what they already have.

(b) They train less but more often.

(c) They've reached the point where they don't need to put much energy into exercise to get good results.

(d) They've lost the spirit.

(e) They've got superior genes and thus can achieve more by doing less.

I suspect it's a mix of (b) and (e). Some people obviously have bodies that enable them to build really good muscle mass if they train right. Unfair, it might seem, but I would stress that the beauty of lifting weights is the lifestyle itself and not only the effects it has. Work hard, eat a lot, and challenge your mind. Choosing a specific way of life and following it through should be part of every man's destiny.