When you say "more of", you are talking about proportions - percentages. When the value that you're getting more of isn't fixed (i.e. two different incomes), then someone can have more of their value while that actual numerical figure is smaller than another person's.
For example: If Bill and Bob both make $50,000, and Bill pays $10,000 in taxes while Bob pays $5,000, then Bill is paying both a larger value and more of his income than Bob. However, if Bill makes $50,000 and pays $10,000 in taxes while Bob makes $100,000 and pays $15,000 in taxes, Bob is paying a larger value while Bill is paying more of his income. The impact to Bill's life is greater than the impact to Bob's life - Bill is sacrificing 20% of his income, while Bob is only giving up 15%.
As a further confounding factor, after a certain point, additional wealth or income is essentially meaningless. For example, Michael Bloomberg is worth about $60 billion. You could take 50% of his wealth and he'd still be in the top 10 richest people in the world. His lifestyle would not change. There is nothing that he could comfortably buy at $60 billion that he couldn't also comfortably buy at $30 billion. You can apply this at a smaller scale for income taxes - essentially, your effective tax rate should increase as your income increases because it is easier for you to live on a slightly smaller percentage of your much larger income than it is for a person of lower income to do the same.