I agree with you with the fact that a lot of smaller guys can lift a good amount of weight. Look at the elite totals for guys in the 148 lb weight class or 165 lb weight class:
Pretty big numbers. I guarantee you most of those guys are not under 10% bf or maybe not even 15%, possibly even higher. Bodyweight is a bigger factor in the lower weight classes where you would have to worry about making weight. Some of these guys might stay leaner to avoid moving up a weight class.
Another thing is there is definitely a performance correlation between bodyfat/weight and strength. It might be in a gymnasts advantage to be lean, but I will argue that there is a performance advantage to be had for strength athletes that are not excessively lean. Cutting weight, worrying about carbs/calorie intake, and other trivialities takes away from training time and more important things.
There is also a big difference between a guy benching 300, squatting 400, and deadlifting 500 at 160 vs a guy at 250 putting up the same numbers. The 160 lb athlete is probably a lot better at his sport than the 250 guy putting up the same numbers. Compare the 160 lb guy to a 250 lb elite level lifter who is benching well over 400, squatting over 800, and deadlifting something else. There is some leverage issues and stuff, but at the end of the day the bigger guy is generally stronger regardless.
I think Eric Cressey put up an over 1400 lb total at 165, as an example of a smaller guy who is pretty good at what he does. I don’t follow powerlifting too closely, so I don’t have any better examples. A lot of bigger guys can’t put up those numbers, but they aren’t at the same level.
I think some guys also just like to lift and eat, and don’t give a shit about their weight/bodyfat. I don’t see anything wrong with it, its just not a priority to them.