Well, yes, that would be true that Ichigo would be quite large--if he was dead. However, since his soul still inhabits a living body, it still feels a connection to that body, even when it is not within it. Therefore, he's still at a human size. His ability not to control or sense spirit energy may also have something to do with the fact that he is not completely "dead." Aka, that he still belongs in the real world.
But, you still may be correct on it not being based on spirit energy. Perhaps you are projected the way you wish to be in Soul Society. After all, we don't know what any of the shinigami looked like when they were alive. The only insight we have is that young cockatoo (or parakeet or whatever) boy from the beginning. He was projected in Soul Society the same way he was when he died--but, he's only one person. Unless we could cross-reference it could be assumed that he wanted to remain that way because that's the way his mother remembered him.