I have never seen a feed trained LMB stay only on pellets when there is live prey around. They start eating natural food and supplement with pellets. In that situation I would be surprised if there is an upper limit on size other than the fish's genetic potential. IIRC AM LMB pellets have the same nutrient content as RT (it was based on lab analysis of RT contents). There are some very big LMB in Calif and elsewhere that get that way feasting on hatchery raised RT plus TShad.