A pec is skin over muscle, when you start to get excess fat or breast tissue it's a boobish. It's partly genetic, and smoking pot can cause it too. Most girls like as little fat there as possible, no matter what they say. I have to work out to keep things in the shape I want, if I slack the boobs come after me!!
It's going to happen if the estrogen level gets high in a man's body. This can happen if the % of body fat gets to high and the man doesn't do any strengthening.