There is a 3.4GB offline version of Wikipedia. I have a copy, and I'm going to try to turn it into an ebook.
I've been looking at it today. The text by itself is only 400MB, and I should be able to cut that in half (or more) by removing most of the formatting.
The images take up the other 3GB. I expect to be able to reduce this to 1GB by abandoning the highest resolution images and by eliminating the duplicate images. I might get it even further.
You can download it as a torrent
here. You will need a bittorrent client.