not by much, go into player settings, andrid tab and look at the optmization section in other settings. make sure your using the .net 2.0 subset and experiemeant with stripping levels. There will always be overhead when using a full game engine, since the game engine its self and the content you created are both part of the apk. Also 8mb isnt that bad, even over 3g networks the cap for mobile downloads is like 50mb.