All's I can say about the sheltie are they are a little dog that get thier hair from standing under a collie as it gets groomed.

Now all you Sheltie owners know I'm just kiddin right. lol I don't want to offend sheltie owners!!!

Seriously though, Size in shelties come from the various pedigrees the breeders use. Its the same way in collies. You can perfect your line to have smaller collies ranging from 50 to 70# or 60 to 100#. Depends on your pedigree line.