Why was Australian allosaurus smaller than American allosaurus?

The Australian Allosaurus wasn't actually an Allosaurus, even though it was closely related to them. Its actual name is Australovenator. Since they were a different species, there isn't anything strange about the fact that it was a different size.

Australovenator was 20 feet long, 6.6 feet tall at the hip, and weighed about 1,000 to 2,000 lb.