Why is attacker revenue in the selfish mining disproportional to its hash-power? It's not clear to me why attacker can gain more revenue if it does "selfish mining" attack than its revenue when it behaves honestly. 

I'm aware of this paper.
Selfish mining is based on delaying the propagation of the blocks a selfish miner (or pool) has mined. Following such approach, other miners continue mining on the top of an old block, wasting their time and  hash power. 

There are different situations depending on how many blocks a selfish mining pool is able to mine in a row before any other node mines one (all the possible cases are described in the paper you cited). Moreover, selfish mining also requires fast block propagation through the network, since in some of the situations they try to take advantage of other nodes mining on the same block as them, virtually extending the hash power of the selfish mining pool. 

Block propagation speed matters since when the blockchain forks in two branches of the same length, nodes start mining by default on the top of the first block they hear about.