Why does .apk file being built consumes so much space?

Hey there! So, I have been trying to develop an android game using unity. After importing some assets such as images i built the game and the .apk file created has more than 20 MB size even before adding half of the assets i needed. I thought the problem would be on the images and their quality.

But when i created a new project and built the project with nothing in it, it still created the .apk file with a size of 18 MB.

My assumption is, default size for an android app created using unity would be 18 MB. If that's a fact, how does people release apps of size less than 10 MB.

I might have gone wrong somewhere or might have misunderstood stuffs. Please guide me through resolving this size problems.

My Editor.Log file shows the details as follows:

I don't know if this could be useful. I am using unity version 5.2.1

Thanks in advance. :)