This is probably a silly question, but why do you suppose the rules for creature advancement has animals increasing to extraordinary sizes when advanced in HD? Granted, not many animals do (according to the SRD), but that some do at all is a little odd.
Eagles, Hyena, Mantaray (in reverse, actually), Monkey, Octopus, Owl, Porpoise, Rhinoceros, Shark, Snake, Squid, Tiger, Whale, Wolf, and Wolverine are the creatures that increase in size.
Some I can understand (octopus / squid, porpoise / whale, rhinoceros, shark, snake), but others are just odd. Wolves and Tigers increase in size, but (wild) Dogs, Lions, and Leopards do not. Eagles and Owls do but not Hawks or Ravens. What about Monkeys - why are they allowed a size increase when Apes are not? And why do Rhinos increase in size while Elephants and Bison do not? I presume Hyenas are incremented simply because Tigers and Wolves are (despite their major nemesis - the Lion - not increasing in size).
It seems almost random, really. Is it perhaps due to some 2e holdout / issue?