There is this reddit thread which provide a lead on the size issue (looks like you already know more or less of it, still your file use yes/no pseudo boolean as-is whereas in doc, they are surrounded by quotes, have you checked your file has been processed successfully ?)
Which version of Godot 4 did you use ? They're releases pretty often so from one to another, some new features or new issues could "popped-up" and have some effect in this matter.
You may consider checking opened issues and maybe fill a discussion and/or an issue on the official Godot engine repository as well as check for generic Android tool to optimize app package size, and also the building process (like get the commands uses and their parameters, check and add some, use better/other tools which can do the same job if they're any)
10-15 MB looks to be more a dream than anything actually reachable to me, not with some comprehensive engine like Godot or any framework which simplify the code, I never seen such small app yet.