It's hard to understand what you're describing here. Do you mean that these guys are walking around with such large pectoral muscles that they appear to have breasts, or do you just mean they're just fat?
If it's the first problem, don't worry about it. It's going to take a very long time to reach that level of development. If you ever hit the point where you think you've got enough chest muscle, you can stop increasing the weight of your workouts and stay there. I suppose one thing you can do to help is avoid anything that's targeting the lower pecs. Decline presses, chest-centric dips, decline flies, etc are all purported to hit the lower pecs. Targeting your lower pecs means that the bottom half of your chest will get thick, which will cause your shirt to sort of "tent" over it, which might sometimes look like moobs. Most of those are exercises I don't concern myself with anyway because I think they're a waste of time (except for the dips), but they are extra counter productive if you're specifically avoiding a thick lower chest.
A lot of it is going to be bodyfat percentage as well. Even if you're relatively lean, you might still carry a bit of extra fat in your lower pecs. Doing all those exercises that then cause the muscle underneath that fat to grow more than the rest of your chest might eventually give you the tenting problem, but getting to a low enough bf% may then correct that. Genetics will also play a role here based on how much fat your body wants to store in your chest and/or where it wants to make your chest muscles grow the most. Can't do much about that except get really lean.
One thing I know about your average gym goer is that they usually spend an inordinate amount of time on isolation exercises. They do a lot of flies. Incline, flat, decline, upside down, sideways, single arm, double arm, sitting standing, running, walking flies. Don't do that. It's probably why they all have chests that appear disproportionate to you, and you don't need it to grow your chest anyway. Do lots of flat presses with dumbells and barbells instead. You'll get stronger and your arm to chest to shoulder size ratio will stay in sync, which will help keep you from looking like those guys. Stay away from the smith machine, pec deck, and chest press machine while you're at it.