The AAB contains the resources of your app in different languages, screen resolutions, ecc...
When you download an app from the Play Store, it will actually download only a part of the AAB, the part that actually is needed on your specific device (based on language, screen resolution, ecc...)
I think that this concept works on Test Internal Track, but not on the Internal App Sharing, because the Internal App Sharing is a lot more "debug oriented", (for example you can upload a debug build, you don't have to sing the bundle with your production key...) and so I think that with the Internal App Sharing you are actually downloading the entire bundle, not optimized for your specific device.
This can be the reason for the different download sizes, but this is just my opinion though :)