Large glomerular kidneys are developed secondarily in migratory fishes as an adaptation to tackle the problem of

Freshwater fishes have to tackle the problem of endosmosis. The osmotic pressure of the freshwater is very low and that of the body fluids of freshwater organisms is much higher. So water tends to enter into bodies by endosmosis. To  maintain the balance of water in the bodies, the freshwater fishes will have large glomerular kidneys.