The size of the .apk file built by Unity is larger than I expected. Is it image resolution, number of scenes, or something else?
The things that take the most impact in the size of APK are the assets (audio files, images, models, config files, icons, and so on) and the libraries (jars and/or dynamic native libraries).
With this information, you can deduce that the more assets and scenes you have, the bigger it will be. Also, with big frameworks like Unity, there will always be a minimum fixed size cost for anything you build.