The Ramchal writes that even though we are on this world to earn our reward for the next world, our toil is finite but our reward is infinite.
(Derech Hashem – First Chelek, Chapter 3:3)
The question is why is considered as if we earned it if it’s so unbalanced?
The answer might be, because when we perform a mitzvah it’s like we are operating a complicated machine that is manufacturing very valuable items. Even though the operator only pushed some button he is credited in producing the valuable items. So too this world is a complicated machine, when we perform a mitzvah great things happen which we are credited for.