The burden on the top taxpayers is increasing
The Wall Street Journal clearly shows that the share of taxes paid by the top income earners has gone up much faster than their share of income. From the WSJ:
See also here for a redone diagram by Paul Caron.
The top 5%, top 1% and top 0.1% of Americans have been getting a bigger slice of all the income and paying a growing share of federal taxes. Average tax rates have come down for everyone. On average, the tax bite on the rich is bigger—except for those whose income mainly comes from capital gains and dividends. The share of taxes paid by the bottom 40% of the population has been shrinking along with their share of income. The tax system narrows the gap between economic winners and losers, but not enough to stop the gap from widening.