The DVR's store the incoming feeds in a different format. They don't compress it that well for both quality reasons and the fact that the CPU of the DirecTV DVR would have problems (especially the older ones) compressing two video's at the same time. It would overheat. So it is stored in such a way the DVR has to do as little work as possible yet allowing as much video as possible to be stored. A balance.
Internet streams are much more compressed, and require a bit more CPU power when playing/decompressing.
I might be a bit off on the 1 Gb/1Hour thing, but it does come close to that. If you have the option, I guess you could monitor it: Don't use the internet for an hour or so and count the bytes coming in when downloading a 1 hour show.
Sure Amazon, iTunes, Netflix, etc. can use different compression and frame rates, etc., but I thought we were talking about DirecTV's On-Demand content and the affect of these data caps on it's usage. DirecTVs On-Demand has to stick to pretty much the same type of compression as what they are using over the satellites, or else I don't think the DVRs would be able to handle it. So like I said with DirecTV's On-Demand content you are looking at 4-5GB/hr I believe, which will eat up those data caps pretty quick.
If we aren't talking about DirecTV's on-Demand, but instead all these other providers then isn't this thread in the wrong place?