How to get bigger bass?

[PondBassin]

Hey guys,
I'm new to this site, so if this thread or anything like it has been done before, I'm sorry.
But anyway, here's my dilemma. I have a pond in my backyard that was stocked by the previous owners with bass, various sunfish, some channel cats, and some mosquito fish (a sort of baitfish for the pond). The problem I'm having is this; the bass in the pond are very small. They have been in the pond for about ten years now ( I've fished the pond for 5 years) and I have seen very few bass exceeding the 4 pound range. The previous owner told me to expect this. The pond is pretty small, and I believe there is a sufficent amount of food in the pond. I also know for a fact that the pond is not overpopulated. The fish are never taken out of the pond and are always released back. They are all northerns, no Florida's or mixes. What I was hoping to find on this site was a way to beef up the size of the bass? I'm not sure if this can be done, as we don't want to restock the pond at all. Should we try and add a species (although I'm afraid this would set off the food chain)? Is there a sort of bass feed that the bass would eat to bulk up? Any answer would be helpful.
Thanks!
PB