We don't know the shape of the universe. There could be nothing further than what we observe. We, by definition, cannot actually know. The universe may simply wrap around itself. The universe may have finite real volume due to big bang inflation and it simply can't be approached

Fact is, far enough in the future inflation will be such that our view is vastly smaller than it is now. Their universe will be smaller because there will be no way for them to access what inflation has put beyond light and causality.