My 2 cents worth, I think its a combination of things, the first being genetics in some areas. Some lakes just have bigger fish because the gene is in them .I think the biggest controller of how big the fish are is population. If there are a bunch at the table then there will not be enough to go around to fatten everyone up, Here in the south our fish get a longer growing period due to the water temperature staying above 55 degrees longer which means they actively feed longer needing more food(shad) ON the big G I am convinced that there are at least 3 types of schools . There is the dinks that stay around the bottom and close to the bank/rocks for protection, then there are the mid sized schoolies who travel in large schools or stay around a place where there is a good food source , finally there is a big boy school that runs together where none of them will be less than 15 inches. I base this on having seen all of us catching the schoolies most of the morning and all of a sudden 3 or 4 of us will get into a really big one usually within a few minutes of each other then they are gone and back to the normal 10 to 13 inchers One thing I know is they are a lot like us they want to be comfortable and they want to be able to eat when they are ready . They will go out of there comfort zone temperature for a few minutes to eat then once full back to the cooler water.