They're using an older codec. Sorenson, to be precise. It just doesn't give you the size:quality ratio you get from the newer codecs. That's never been Sorenson's game; its strong points are its high quality and low playback requirements.
It's not QuickTime's fault. QuickTime is on top of the game here. It has MPEG4 and H.264, the two best codecs out there, period. WMV3's quality is good (somewhere between MPEG4 and H.264, IMO), but its system requirements are about double that of H.264 (which, in turn, is naturally a bit more demanding than MPEG4). I can play 1080p HD MPEG4 content on my 1.25GHz Mac mini with 256MB of RAM; Microsoft lists a 3GHz P4 with 512MB of RAM as the minimum requirements for 1080p WMV playback on Windows.