HD DVD was 15/30 gig versus Blu-Ray being 25/50 gig storage, yeah. For mainstream use that was far more important, so that played a large part in why it won, though a lot of it was also Sony making the PS3 Blu-Ray-based and giving adoption rates a huge boost as a result. In terms of actual video codec support though the two were identical: H.262 MPEG-2, H.264 AVC, and VC-1.
As for how much content you could store on a CD, there were a lot of video resolutions supported in between 240p and HD, and H.264 can compress video quite a bit while still looking decent. For the die-hard video enthusiast, not much of a draw. For someone wanting to distribute stuff on the cheap, especially in poorer areas and "emerging markets" where SVCD players were (at the time) still commonly sold? Huge draw.
EDIT: Also of minor note was that the video (but not audio) formats from previous CD-based formats were completely compatible with the HD DVD standard, meaning in a pinch someone could just take the existing video from an SVCD/CVD release and drop it into a HD DVD. Of course why one would do that is a valid question, but nonetheless the standard was set up to allow it. For whatever reason.