There are thousands of small miners. They find blocks just as fast as a miner who has thousands machines. If the pool under-pays the small miners, the small miners contribute their time to the block finders most of the time.
"Small miner" argument doesn't really work here. Pool pays him like he's finding block every 1-2 days (6-7xpms/day) but in his words he rarely finds 9ch, so he doesn't really find blocks that often. It means pool hugely overpays him, probably out of pocket of people who find blocks.