What does it do with disk space? – Google Play Books & Audiobooks Review

It’s a nice convenient app, I can upload a book from PC and continue reading it on iPad, but why does it consumes so much space for books? When I start to read a book that weights less then 2 MB, this app downloads 800 MB! 800 MB OF WHAT?
Review by Eisenspiel on Google Play Books & Audiobooks.