There's lots of room for more people. Canada has what, 3 people per km^2? And many of the empty regions are at least somewhat habitable, given the same sort of technology that would let you settle Mars.

OK, Mars is unlikely to have the same blackfly problem as Canada. Insufficient lift for their little wings, you know. Moose wearing marssuits may be a problem but I think the mammoth problem is overstated.

Anyway, we seem to be close to the inflexion point on world population growth. In a century, the problem might be producing sufficient kids to keep the economy going [1].

1: Actually, it will be probably be something whose roots are staring us in the face today that we're overlooking.