In the end the engine is a tool and it is how you use it.
Unreal Engine is not as mature as Unity when it comes to the mobile side of things, but they are getting better every release and the community has released some good plugins to assist you with mobile development.
There is documentation on reducing APK Sizes: https://docs.unrealengine.com/latest/INT/Platforms/Android/ReducingAPKSize/
You would need to search the forums to find the plugins that will help you and ultimately you would have to make the decision for yourself. If I was in your shoes and I wasn’t tied to a specific platform I would evaluate both engine by doing this:
- Choose a simple concept to test, let it include a menu, a simple level with some mechanics, and maybe a pause menu
- Build the concept in Unreal Engine
- Build the concept in Unity
- Compare how easy/difficult it was, size of APKs, ease of packaging, ease of use, etc.
- Choose the engine that meets your requirements
I don’t do mobile development for specific reasons and I chose Unreal Engine because I have access to the source code and I can package the same project for Windows, Linux, and Mac quite easily … so it is my preferred engine of choice.
Good luck with your project and I hope you find the answers you are looking for.