I think I've got it. Basically, anyone who's passed email verification and made at least 1 post can create 75x75 avatars. Anyone who hasn't passed those two normally can't create avatars, with some exceptions which allow 48x48 avatars. The exceptions presumably allow 48x48 because that was the old default.

@DoubleGJ: it looks like you've not verified your email address but you're lucky enough to fall into one of those exceptions. Verify your email address and you should be able to upload a 75x75 avatar.