No secret..... a large deep rib box gives a chest a foundation to build upon, an extra advantage when adding muscle mass. Some are born with that deep full rib box, and look impressive even if never working out. Having a somewhat muscular ab area only increases the chest being impressive. If starting young enough, the cartilage between the ribs/breast bone is still in a elastic state of flux and can be encouraged to expand a bit. Programs of breathing squats pullovers, etc have accomplished this factor quite well. Depending on the individual, around the age of 22-24 the elastic state of the cartilage begins to set and harden. Not a case of the rib bones growing (can't do that), a case of the still plastic state of the cartilage stretching/expanding out a bit....if young enough when starting serious training..
Benching hits both the upper and lower pecs, though not in equal proportions, which only insure an unbalanced development. When working the bench, might suggest adjusting different hand spacing to see if you can come up with different results for lower or upper pecs. Seen a few guys use close, middle and wide grips when benching, in a single workout. Usually three sets each grip position.
As mentioned before, lifting the knees up and crossing the ankles give a more focuses affect to the chest area, taking that extra body English, including the legs to a lesser degree, out of the movement. . Tends to keep the lower back flat on the bench and more focus on the pressing motion of the bench. Seen many pure high volume BB'ers pushing around the 220 to 230 range when benching, with much better size and development than a lot of the 400+ heavier bencher's.
Dips are an excellent exercise for the whole chest area. With serious attention to dips, rather than doing flat benches, many make excellent improvement, where improve has been lacking. or little for years on the pecs. . Even bring the knees up and crossing the ankles (Gironda styes) allows more attention to the whole region.
Good Luck