How to reduce initial build size for Android

Hi,

I recently published my first app on various plarforms including Google Play...(get it here if you like ; ) :-

https://play.google.com/store/apps/details?id=com.KuXueGames.FlagzNew

I got generally good feedback for it but one major criticism I received from friends and 1 or 2 users was the size of the file - 31M for essentially what is just a quiz game.

As an experiment, I opened a blank project in Unity and found that it came in at about 20M before the addition of any assets whatsoever.

I've had a good look round the forums but it seems there is no way to reduce this, can anybody confirm if this really is the case?

Thanks y'all.