You can’t do much better than that; that’s the size of the audiobook! For what it’s worth, I also used Storyteller on Wind and Truth, and got it down to 1.2GB by using the OPUS codec with a 32 kb/s bitrate.
So I'm trying to design a progress sync protocol. My current idea is to just use several words from the text itself to unambiguously pinpoint the position within a section (chapter).