Hey man,
If you think you see size differences that ofte shift to big to small, small back to big...I would guess it's in your head. You are probably thinking about it too much, all of us gyners devote our minds to this problem and it can play with ya. Anyway you are only 15 years old and no matter what you'll have to wait 3-5 more years to know if it's the real deal. You are at the prime age to get that really common temporary puberty induced gyne, i wouldn't flip my lid too much. shit i'm 20 and I am just now starting to figure out " what the eff am i going to do?" So, you have a good start and you'll probably be fine. Anyway, are you fairly overweight? What is current height and weight?
It's fairly easy to figure out what you need to do for fat loss and muscle build up in the chest region. To be very simple and obvious, you need to do atleast 30 minutes of cardio 3-5 times a week. Push ups are old fashion but still they are one of the all around best exercises for building the chest area. Watch it though, people with gynecomastia shouldn't build the pectorals too much because it will force the gland out more than it already is,,,making life generally worse.
