why does this happen when i scale this model up?

why does this happen when i scale this model up?

the skin may have a bone which has been deleted. check the bone names against nodes in the scene. Bones in a skin modifier are handled odd, and even if a bone is removed, the index still remains. If you need to, repaint the skin from scratch.
Alternately, sometimes you can add another bone to fill the missing index. You can then export the model and check the skin-bone section for weird numbers, zeros, or bones without proper names.
Oh, I see this is on import. Check that the file being imported does not have bone name errors. You'll possibly see bones named unlike nodes in the model. Depending on what made the file, you may find bones that reference the object base as some other word.