90% sounds to be too high an overhead. At 435 words per article [1] and 3553578 articles (English only) [2], it has ~1.5 billion words. At 5.1 letters per word and one extra for space, it comes out to 9.4 GB. With 27 GB uncompressed, even that sounds like too much overhead for markup, but maybe.

[1] http://en.wikipedia.org/wiki/Wikipedia:Size_comparisons [2] http://en.wikipedia.org/wiki/Special:Statistics

"(This is probably the one you want. The size of this dump is 6.07 GB compressed, approximately 27 GB uncompressed as of 2010-09-20.)"

That's just the HTML, no user or talk pages.