So, the unity engine it self is ~12MBs. This differs somewhat depending on what version of Unity you’re on. Android normally unpacks the libraries when put on a device so that would cause your application to more or less double in size if your assets usage is low. If you are using Unity 4.6.x you will also see an increase in size since that version by default include two versions of the engine. One for x86 and one for ARM. You can build separate APKs for each architecture by changing the “device filter” filter option in player settings.