The amount you earn back from the post you upvoted depends on a number of factors - how much the post made during payout, at what point in time you upvoted that post, the number of votes before you upvoted, voting power, and so on and so forth. Take a look at this article to learn more about curation rewards.
Bits like this are the stuff that makes the learning curve of this place steep.
It is the same questions as "Why do my rewards get smaller within the seven days period?" or "Why do I get so little SP for my post, if you told me it was 50% of the total?"
Man, sometimes I get confused too...