When you write an Android app using Kivy and Python you have to include some Kivy and Python libs in the build, Kivy itself is 5-6mb so depending on how much data you include in your app the size will vary. Each build has an entire Python interpreter as Brousch pointed out.
After you have built an apk, if you look inside the file you will see the different sizes of each file in your distribution.
If you look here there is a guide to the basic layout of a distribution.