this is beacuse when you are packing app you aren't packing only your custom code (controlers, views, models,...) but whole rhomobile runtime (rhomobile libaries, ruby runtime, integraded webserver,...). You could preinstall rhomobile shared runtime on device and than package your app in shared runtime mode. This would decrease your app size dramaticly but if i'm not wrong this is only possible on motorola devices.
Hope this clears things out.
If you require use of the Motorola Webkit you can decouple the RhoElements webkit from your app, per Andraz's commentary. However, if you plan on using the native browser to render your app then the Motorola webkit is not required. In this case ensure the Motorola_browser under capabilities: is not included in your project's build.yml file. An non-optimized Android project without the webkit should be approximately 2MB in size.