Glimmer 2 has greatly reduced the size of Ember applications.
It is possible to write an Ember-CLI addon to replace Uglify with Google Closure compiler. So far, there has been little interest in this. It looks like someone did do this once two years ago:
If anyone is seriously interested, they can try to update it.
Have you tried manually minifying your Ember application with Google Closure Compiler instead of Uglify? Does it give any better results?