Answer by emongev

The apk when splitting obb files loads into the apk whatever is in the initial scene, so its probably due to the fact that your initial scene is filled up with things. My recommendation would be to have an initial empty scene (probably only a texture that says loading) where you use the obb downloader only, and forwards to your original first project scene when its done loading. That way only the texture will increase the size of the apk file (there are various other things like scripts and shaders that may go into the apk, but its probably images or other stuff thats increasing the size of the apk).