Are you by any chance leaving Google Docs pages open for a long time? Safari runs great on my machine, with windows and tabs open for weeks at a time, and the only problem I ever have is with Google Docs, because Google Docs is a gigantic memory hog. If I leave a Google Docs page open for a day, it usually ends up consuming several gigabytes of memory all on its own. For a company whose focus is the web, I don't know why Google can't get their memory usage under control.

(Caveat: I haven't actually tried leaving a Docs page open for a day in a while. I stopped doing that when I discovered how badly it behaved. It's possible Google's fixed it, though I doubt it)