Recall that the marginal utility of money diminishes when you have more of it - when you have a lot of money it's easier to turn it into even more money, and vi

Recall that the marginal utility of money diminishes when you have more of it - when you have a lot of money it's easier to turn it into even more money, and vice-verca. It's not linear. So 20$ difference has exponential not linear influence on "being rich".