Another way to think is that you are not punishing them more, you are punishing them equally if you do it by % of "richness" Meaning that if a fine means that someone struggles to eat the last 2 days of the month, that should apply to everyone, no matter how much you make, it just happens that you have to take away more from the ones that have more to get the same effect. It also makes that those that are not soo well off are not disproportionally punished.

Lets make a imaginary extrapolation to see it in other light, lets say that there where a second human like species here on earth, but they live until around 1000 years instead of 100 but they experience time 10 times quicker, so for them 10 years feel like 1 year to us. Would it be fair to do the same time in prison for them than us for the same crime? what feels psychologically like 10 years and 10% of our lives would feel to them like 1 year psychologically and will be only 1% of their lives, and if we started to put harsher penalties to account for them, they still will end up with kinda light sentences and normal humans would end up with absurdly high sentences.

If the idea is to punish so it is not done again, sentences have to be proportional to the one receiving the sentence to be effective if not you are over-punishing some are under-punishing others.

OFC this is with the idea that fines are punishments and not just paying for what you broke kind of deal, for example if it is a fine for parking in a place with a parking meter without paying, the fine should be what you did not pay plus a little bit extra.