I also think you're taking his comparison to literally: I think what
@Umbran is saying is that dramatically different races with dramatically different physiologies are going to cause balance problems.
I mean, it's the same dilemma that keeps coming up with the ASI discussion: it can both be argued that +2 to an ability score (which translates to a +1 die modifier) is a pretty minor difference that could be safely ignored AND it leads to a dramatic bias in the characters that players actually want to play.
So what's going to happen when you have both bunnies and rottweilers? Well, to maintain balance, let's make the bunnies more ferocious, and the rottweilers less ferocious, but leave a little gap to maintain the distinctiveness of the species. Ergo: "Golden retrievers with funny hats."