BookI APP
This app is the smallest-I-know and fastest-I-know offline reader for Wikipedia (and other books), with his own "squeeze-all-in" format. Some example, the entire Wikipedia in English (6764322 articles) is about 18GB. Still big, but not so big for modern phones... plus, all titles of the articles are indexed, for a fast search.
The downside? The interface is minimal and focused mostly on reading text (that is what counts, no?)