Based on what I read from the White Paper, the rate at which your curation rewards increase in proportion to your steem-power is nonlinear.

They point out that someone who has invested to have about 200 steem power gains a very significant amount of reward increase compared to having just 5 steem power or 0.

But increasing from, say, 50,000 to 100,000 steem power is not going to provide such a drastic difference as those first few hundreds of steem power do.

It's clever! Sometimes the underside of Steem resembles a MMO computer game more than a social platform.