1.53 is the Vista update-1.51ish was a "normal" update. Not sure what specifically has changed from 1.4x. 1.5whatever is also up on SDC, and it will properly show in SDC as somewhat less than 1.4GB. In Impulse, it doesn't always properly show how large the update is until you start the update.
My guess would be ~200MB, but it may even be less than that.