What is APK file size difference in each unity version

I was using unity 4.3 and built an APK of 15MB but i upgraded to unity4.6 and now the APK size is 20MB. I want to know that how can i reduce its size but with keep working in unity 4.6.

@john1 @clark @gamejam

Because Unity have added more features to the engine… realtime GI, new particles, 2d support, physx etc. Typically more features require more code = more filesize.