Looking at the various galaxy maps in the rulebook, Tyranids most assuredly haven't eaten a quarter of the galaxy.
http://wh40k.lexicanum.com/mediawiki...incursions.jpg
Compare that, to, say, the Orks.
http://wh40k.lexicanum.com/mediawiki...s_activity.jpg
And those are only MAJOR Ork planets, and not even all of the major ones.