An hour of HD video can burn as much as a Gig. I'm pretty sure the VOD system is just basic streaming, so when you back out it stops downloading. I don't think it even buffers.
I don't use VOD (useless on my crappy DSL connection), but my router tracks per-device usage and my 630 uses just under 20MB of data every month on average. Not sure what that is, likely the "call home" stuff since it's not plugged into a phone line.