How come Democrats have held larger majorities in the House than Republicans do today?

This was the biggest gain by either party since 1948. According to exit pols, the reason for the vote was The Health Care Law, the way it was ped, the way it's being presented, and the latent effects(higher cost of premiums) for those who already had insurance. We find out that seniors are having their services diminished, that those on Medicaid will have their choices made for them, that not all of us will be able to keep our present coverage, that Congress exempted themselves from the rules(again), and that it does little to actually improve care for many. That might be considered a mandate since it seems to be the issue which drove the change.