basic 19 megabaytes ithink that is too much
I'm not sure where you get the information that it is too much. Have you looked at the sizes of games made with Unity compared to non-Unity games? Do you know how many megabytes / gigabytes Android games currently are? For typical graphical games, somewhere around 50-100 MB is the norm, with high-end games crossing the gigabyte boundary.
Five years ago the average size of games was about 40 MB, but since then Android has bumped the size limits of programs so average game size has increased.
The current cap is 100MB for the APK, plus another couple gigabytes for data if you need it.
For the Unity libraries for both Android chipsets your size of about ~18MB sounds right. Unity's core libraries were about 8 MB last I checked, and Unity automatically builds for both versions of Android. That is, it includes both ARM (the dominant processor) and x86 (used in netbooks and some Chromebooks) inside the APK file.
You can change that -- though I wouldn't recommend it -- in Build Settings | Player Settings | Other Settings | Device Filter. Change it to only build Armv7. Doing that means you remove anybody using x86 devices from your potential customer base, but at least your .apk file will be slightly smaller.
The core libraries are doing quite a lot. It includes all the runtime libraries for the engine, all the rendering code, effects code, networking code, memory management code, world manipulation code, AI and pathfinding and animation and audio and physics code. Plus it has all the stuff necessary for the .net runtime (the stuff all the C# and UnityScript and Boo code compile down to. Compared to other systems, an executable size of about 8MB is rather small.