Watching on a tablet may use a lower resolution, and therefore less data, than a HDTV. Check to see if you can set the resolution on the tablet to something less than 1080P, like 720P or 480. If the tablet uses the full resolution then it will use the same amount of data as the HDTV at full resolution.
Nicholas is correct; HD has higher resolution and as a result is a much larger file size than a Standard Definition video. If this is done during your metered hours, you'll see it consume quite a bit of your data, try reviewing your resolution settings.