I've had steam since Apr 3, 2004 so my collection is quite large. The only reason my collection here is smaller than the 360 (retail + xbla) is that the frequency at which I buy games is directly related to how good my computer is at the time. And this collection stretches over the lifespan of a few computers. Thanks to Steam sales it also has far and above the most unplayed games I own on it.
(comments to be added sometime in the future, maybe)