It seems to me that little fish are more difficult. They would burn faster, have to be watched and timed closely. A large fish just needs a bigger pot and more oil.
Perhaps we place more emphasis on frying large fish so as to reduce crowding amongst small ponds. Perhaps the fryer of the big fish has been lobbied by all of the medium-sized fish, who then vie for the position of largest fish. That goal is extremely short-sighted, however, as once one achieves big fish status, one must be always on the lookout for status-seeking chefs bearing large pots.
I guess the moral of the story is that frying is inherently unhealthy for all involved (except, possibly, for the fryer), and we should instead grill all of our seafood.