My bet hound is really small....?

My bet hound is really small. She is 8 months old and doesn't seem to be getting any bigger. Her parents were both really big, probably about 70 pounds each. Her and her sister were both the same size, so I am pretty sure she wasn't the runt. So why is she so small? Do bets tend to grow slower? Will she start growing again once she goes into heat? Or is she just going to be small? She only weighs about 40 pounds.