An interesting question. I haven?t given it any thought before. Here are my thoughts which come to mind now but I don?t claim them to be authoritative.
Organisms have the potential to grow larger in water than on land because of the support given by the water. There is no need for such bulky strong bones as would be needed on land. By the way, I think many of the large dinosaurs lived in swamps or shallow water where they benefited from buoyant support.
Large organisms have long gestation periods and few young so population numbers are more susceptible to increased predation. In an environment where there is a moderate level of predation, reproductive rates must be high enough to stand this loss. Potentially, a very large organism would be immune to normal predation simply because of size. In order to select for greater size at the expense of lower birth rates the organism would either have to have a strong defence against predators or there would have to be a temporary removal of predators from their habitat.
Maybe some environmental conditions gave such a window of opportunity during the development of mammals that didn?t arise during the evolution of fishes.
An afterthought. Maybe there is an upper limit to the potential size of a cold blooded organism such as fish, that is smaller than the potential upper limit of a warm blooded organism? Warm blooded organisms have arisen relatively recently on a geological timescale so maybe this is the first chance organisms have had to get this big. Maybe there will be more and larger organisms in 50 or 100 million years time.
Keith Avery - former met-man at South Georgia(72/73) and Adelaide Island (73/74)