Actually I got to thinking about this at work (don't ask why, it just popped in there) and I realized the Andalites live for a very long time but reach maturity very quickly. If an Andalite year is like 6 earth years and Ax expects to live 200 andalite yers (when he says that he could live to be 200 years old he doesnt say "earth years" so I figure he means Andalite years) simple math says that's about 1200 years. Now given that Ax is born not so long before Tobias, who was 13 at the start of the series Ax would be about 2 or 3 Andalite years old, yet is already an adolescent.
If Andalites reached sexual maturity at, heck lets say 5 years the've got 195 more years to keep on reproducing and reproducing, their birthrate might be slow and their term long but didn't the Ellimist have 3 surviving Andalite children, and others who didn't? That was the prehistoric andalites, they would have technology by now that would better keep most infants from dying at birth or later from illnesses, and while this might explain why present-day Andalites had restrictions on how many children were allowed to be born outside of war (As stated in the Andalite chronicles) in the thousands of years before they reached that point in time with their natural predators wiped out they'd overrun their grazing areas and have to live in the harsher areas or die of stavation.
So in short, Andalites might have a small controlled population now, but you'd think at some point sheer numbers would have forced their ancestors to explore more of their world . . . then again, maybe they fought a lot of wars, they have a proud warrior tradition after all. Even so I think they'd have to have migrated to various parts of their planet early on, so they should have evolved at least some differences . . .
So yeah, this is what came to me at work . . . you can tell how much I love my job when I start considering the lifespans and evolution of alien species -_-