If the other babies are light colored ones, it might be not so much that they don't have as much fur, as that the light colored fur is not as easily visible as the dark when it is first coming in.

There are definitely times when some babies get their fur a little sooner than others. Big thriving babies do seem to get their fur faster than littler babies that aren't doing quite so well.

Since it isn't unusual for babies to have a full coat of fur by day 8, I'm more concerned that there is something wrong with the rest of the litter, than that the big one is a super mouse.