The size of fish is very similar to that of the Whitetail deer. Genetics, food, and age.
With respect to the LMB in California, the giants are actually a cross of a Florida Strain and Northern Strain LMB. The F1 generation produces exceptionally large offspring... Each generation after that produces fry that will be smaller in size. This is a genetic phenomena that can be easily seen when crossing a Tiger and a Lion...The offspring are bigger than either the Lion or Tiger.
So the window to catch the Giant LMB of California maybe closing in the current lakes that are producing the 20lb + fish.
The SMB of the Great Lakes Region are getting bigger because of the Goby. They have an umlimited supply of high protein food that allows the SMB to maximize their genetic potential....similar to a whitetail deer that has unlimited food and low social pressure.