I think that there are various factors in a mitzvah and that a mitzvah and schar is a lot more complex than just “Hashem said do ____ and you did it so you get a check. Theres different levels of how well you completed a mitzvah and how excited you were to do it as well as how difficult it was. In short I believe that there are many factors involved in the sum total of how much schar you will get for every mitzvah you do.
So for example if a rich person gives $10 to tzeddaka he will get schar for helping the poor person with $10 of his needs. However since the mitzvah was easy for him he will not get schar for difficulty. So the sum total of schar this mitzvah wont be so big.
If a person who himself is poor gives $10 to Tzedaka not only does he get schar for helping the ani $10 worth. He gets a lot of schar for the sacrifice of giving those $10 when he himself is hardly making ends meet. (The sacrifice may be even bigger than the actual schar for helping the ani $10 worth) So the sum total of this mitzvah will be a lot greater than the rich mans mitzvah.
However 2 people who are well off and all other factors being equal one gives $5 and the other $10 its certainly a bigger mitzvah to give $10 because it does more for the ani’s bottom line.