Total income isn't the question--it's a matter of disposable income. If a poor man earns $20,000 and spends $18,000 of it on food, shelter and medical expenses, his disposable income is $2000. If a rich plumber makes $250,000, he might spend $100,000 of it on necessities, but he still has a disposable income of $150,000. He can pay taxes at a rate several times higher than the poor man, and still come out way ahead.

That's the whole idea behind graduated income taxes.