It says that (in general) Hashem lets a righteous person suffer in this world to cleanse him from sins (so he will have no more sins in the World to Come), while Hashem lets a wicked person prosper in this world so that he will get all his reward here and will suffer in Gehenom.
I have two questions on this:
Let's say there is a standard unit of pain. So in this world, one can only have, say, 5 pains a day while in Gehenom one can suffer 5 million pains a day. Moreover, let's say that a sin is punishable by (say) 5 pains. One does one sin, he gets 5 pains, two sins - 10 etc.
Even if one can't have a sin->pain ratio, as a lot has to do with intent, the idea is there.
Now, if someone sins a million sins, he deserves, say 5 million pains. Why is there a difference if he gets this pain down here or in the world to come?
The only difference should be the time Moreover, even if you can say "a million pains at once is harder to deal with than one", that should be taken into account when calculating the punishment.
Moreover, if Gehenom is in reality qualitatively harsher, then it seems not fair that a wicked person should have to suffer more than a Tzaddik would for the same act.