According to the write-up on Wired (http://blog.wired.com/business/2009/01/magnolia-suffer.html), Ma.gnolia also took a snapshot of the page being bookmarked. This may account for the size of the database.

Yeah indeed... .5TB is huge for bookmarks (title, url, tags, description). I have never had the chance to build anything this big but if you imagine the amount of text you could fit in 500GB, it makes you wonder.

I gave a look at your comments and found this one which replies to my question perfectly (I didn't follow that thread, discovered it right now): http://news.ycombinator.com/item?id=459000 -- thank you for sharing your experience!