Oh that, yes it's just down to the encoding and the rate they used for the particular piece of video. Most muppets don't even realise you CAN encode things at different or even variable bitrates and just set it to be for a particular size. Doesn't help that they chose MPEG-2 as the standard for DVDs when there were already better codecs available either, as it's simply lapsed over into broadcast video (freeview/sky/etc) now. Once again a choice of the known over the applicable.