As far as I know, one is limited to 5 MB, I'm probably mistaken, however, there is a limit, unless one builds and run App Inventor 2 locally.
I remember that someone once detected the solution for similar problem. In the hindsight, it was a trivial one, too much resource files that weren't deleted.
It is easy to forget, while one experiments with one's GUI, that all those previous attempts will be packed in application, thus influencing the resulting size.
Perhaps, to take a look at 9.png format that google provided for Android, in the case you have buttons for different screen resolutions? Or maybe some .pdf file or similar that you can get around?
I hope I was of some help.
Best regards