Well the worth of an upvote is dependent on two things which are the person's steempower and the voting power the person is using. In the case of people who have lots of upvotes but low rewards, it could be one of two reasons

1. The people that upvoted the post have low steem power. To put it simply, the less steempower you have the less a person will get when you upvote them. Right now to give an upvote of 0.01$ you need to have around 100sp so even if a lot of people upvote your post but they have little steempower, it wont add much to you.

2. The people that upvoted the post are using love voting power: Voting power simply refers to how much of your steempower that you're using to upvote a person's post. If the people that are upvoting a post have a reasonable amount of steempower but the voting power they're using is small them you won't get much by way of rewards.

Having low payouts but a large number of upvotes is usually cause by a mixture of the above two reasons.