Well, you see, since we have no idea how life might arise on another planet (we only have a few clues about how it happened here), and we have no idea how evolution would progress on that planet, you can say almost anything about what aliens are like and no one can contradict you.  I think that what he's trying to get at, is that population density goes down with increasing size, so for another larger planet to have the same population as Earth, the density would be lower, so the size would be bigger. Bonus points to the person who finds the most inconsistencies with that logic.