short answer... the physcial and behavioral traits of your dog will be determined to a large degree by its lineage. genetics is the most influential factor. but this is a foundation. nutrition and training will play a lesser role but can help bring out true potential.
but no animal can exceed their genetic potential. under the most optimum conditions, they will reach a maximum level of development.
"real" pit bulls are supposed to be lean and athletic. if you are using internet pictures as a comaprison, you are probably looking at bully dogs (which are not pit bulls despite what their owners say).
~ jo and Gotti Boi (aka Thunder Thighs, aka Jughead, aka The Doo Doo Monster, aka Sexual Chocolate, aka Mister Smiley, aka Sofa King)