Neptune has more mass even though Uranus is the larger of the two because of density. While we measure the gas giants starting at where we can see their visible features, the gas surrounding their rocky cores can be thick or thin just as the rocky cores themselves may vary in size. In the instance of these two planets, Neptune has the larger rockier core with less gas surrounding it, hence appearing to be smaller while Uranus has a smaller core with more gases leading to the planet Neptune being more dense that her neighbor.
