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It's also based on your SteemPower. Curation is best done by those who have already accumulated some SP, large fish, small dolphins and above. If you are a minnow, you should concentrate on posting and use your votes on friends.

Thanks for the advice, I was about to give up curating at all because it seemed so pointless but I see it now. Explained in detail by @shenanigator's new doc :

I’m upvoting posts at the right time. Why am I not getting any curation awards?
The network essentially calculates curation awards based on the square of your Steem Power.
Note: There are other factors in play, but for simplicity’s sake we aren’t going to get into them here. These numbers are approximately correct.

Example:
Bob has 10 SP; the square is 100
Jim has 100 SP; the square is 10,000
In this example, Jim has only ten times more Steem Power than Bob, but Jim will get 100 times higher curation award than Bob, all other things equal.
Double your Steem Power, get 4x the curation award
Quadruple your Steem Power, get 16x the curation award
Increase your Steem Power by 100x, get 10,000x the curation award