I have seen some pretty hefty chihuahuas too. I'm just not real familiar with anything but poodles, yorkies & cocker spaniels.
I had a tiny (less than 5#) toy poodle that was perfectly heathy but then I know that there are large standard poodles which are also fine. It is rather amazing to think there can be that much difference. I know humans come in all different sizes too but usually no more than double or perhaps triple, but in the poodles it would be like ten times the size.