The top earners pay the majority because they're earning the vast vast majority of money and are increasing that majority share at a rapidly increasing rate. It's common sense that pretty much how ever you tax it, whoever controls 80% or more of the wealth will end up bearing the brunt of taxes as well.

Add the fact that most of the whales out there pay a much lower marginal rate than you or I makes it all the more impressive that they carry so much of the load. It's a stark sign of the disparity.