Pond Management

Fish at a 7 acre private pond, and know the owner. Located in Tennesse. Haven't fished there more than once, but my friend and i caught 2 small, somewhat plump little bass 6 inches and 4 inches. Then i hooked the one in the picture, which was 3 lbs 15 oz and 20 inches. I'm wondering, based on the picture and the size of the fishes belly if it was Skinny because not enough food, or a Post-spawn female? If it was skinny because of food source, how would it get this big in the first place? I caught it on June 13, so it could have been a late spawner. Just tell me what you think. Also, just some tips on how to keep the size of the bass in the pond big. How to know what sizes of fish to take out and stuff. Thanks!