Spells – Use the extended person for non-humanoid creatures

Expand person it has at first sight an arbitrary restriction of only being usable in humanoid creatures or humanoid monsters. this is especially important since the SRD lacks spells with the same function that works in other types of creatures. I am aware of Fair power but that adds other things and it's not just an increase in size. Giant bugs is closer, but ultimately it is even more restrictive than Expand person, only being able to increase the size of a few specific insects. Is there a balance reason for this? If not, is there any reason not to allow it to affect other types of creatures (for example, Animals, Ooze or Undead)?