launching unity app from android

hi! i saw an app that the size was 2 mb, but there was a button, and when i press it, the onclick event launch a unity Augmented Reality app...my question is...why the android app is so light? i made a unity app very (super) simple and the final size was 20 mb...and the android app (with the unity app inside) was 2 mb...any help? thanks!