That's wrong. My Oblivion directory is at around 8 & 1/4 gigs, but the shivering isles BSAs total less than a gig. And, remember, I have a lot of mods. My data directory is over 7 gigs now. I also have 1,215 screenshots totaling just under a gig inside there as well. So... yeah, I don't think SI needs that much space. They must just be listing Oblivion's stock requirement or something, unless there's a bunch more added info somewhere that I don't know about. Which I suppose is possible, but I dunno'.
EDIT - So I got up earlier than everybody else today and played some more... this expansion continues to impress. The capital city, while far from "huge", is really, really cool. Just fun to explore, all kinds of neat stuff. There are unique spells, summons, lots of side quests, the usual stuff, just all unique to SI. I'm impressed with the content. You could get well beyond 30 hours with this thing if you like to do side quests, raid dungeons, wander around a lot, etc. I've only carved a swath of about 1/4 of the entire map in my travels, and I haven't really "done" most of what I've discovered... but I can see how much is there and how much there is to do, and that 1/4 has at least 5 dungeons, two towns (aside from New Sheoth), and plenty of random stuff scattered around the wilderness.