It sounds like you have a stock pile of 12" range bass. These bass are stunting at around that size. They don't grow any larger so they feed very heavily on 3" to 5" sunfish but none larger. They deplete their food supply quickly and stay stunted. The sunfish that get a little bigger have no larger bass to thin them out which means they are over populated as well and stunted. I would work on removing about 50 lbs of bass per acre. This would be around 300 bass from your pond. This should allow more food for your bass to grow, enabling them to reach larger sizes to better manage your sunfish population cutting back on their stunting as well.