Awwww! You sound all worried and disappointed!
Presumably, at this stage, she's been checked out by her Vet? What did she/he say?
All I can tell you is Harry's mum - when she was expecting Harry plus 2 others - was fairly big (as Yorkies go!). For her next litter, she only had 1 puppy - and that time she hardly showed at all. So it really does depend on the individual dog, and how many she's having
etc.
Sally + Harry x