The most important factor is the Steem Power of the voters. Each person on Steemit has a different level of Steem Power, and those with more Steem Power bequeath more money when they upvote.

Another factor is the power percentage. For your first upvote, your Steem Power is utilized at 100%. As you upvote more and more, your percentage dips. In order to get back to 100%, you need to wait about 39 minutes. This measure prevents users and bots from spamming upvotes.

I've also noticed that my Steem Power bestows bigger rewards when I upvote posts that have already made a lot of money. On a post that only has $.50, my vote will increase the value by a few cents. On a post already worth more than $1000, my upvote can increase the value by more than a dollar.

Other than that, I'm clueless.