Well, blu-ray movies can be up to 50GB... Unless you want the heavily compressed picture and sound. And honestly, you can't compare the picture of HD downloaded movies with Blu-Ray HD. (I have PS3 and Apple TV connected to 52" HDTV) Not to mention the HD downloaded movies only have stereo sound VS 7.1 HD-DTS/TrueHD.
Largest 1080P movie I downloaded was 28 gigabytes. smallest was 7 gigabytes from torrent sites. For most computer screens, 720P is just fine but I have a 30" monitor so 1080P has to stretch to 200% to fit it.