I think it has more to do with the site's purpose. People come to WrongPlanet for detailed on-topic discussion (or at least they did while I was there; I haven't been back in a while) but people come here to just hang out. In topic-oriented forums it's definitely necessary to cut down the spam, but I'm still not convinced it's about size. SomethingAwful, at least the little bit I've looked at, seemed to be full of inane one-liner type posts. And it's huge.
I would have thought more functional would be a Slashdot-like system for rating posts and posters, so the community as a whole can enforce its standards.