Which is better and Y:
a) There are fewer people in X than in Y
OR
b) There are fewer people in X than there are in Y.

Those are both fine. In fact, they're identical; the only difference lies in the fact that 'there are' is implied in the first one. If forced to choose between them, you might give the nod to the first one, because it's more concise, but both are totally grammatical.