The larger you are the more invested you are in the overall system. At that point you're much better off having a smaller share of a growing pie than a larger share of a smaller pie. Whether this truly works or not, we'll see, but the idea is that the incentive structure is such that people will be encouraged to police their own behaviour.
The whitepaper also talks about "crab mentality". Any time someone is seen to be abusing their power, it is possible for the crowds of smaller voters, as long as the SP is spread widely enough, to pile on the downvotes in response.