Why does the T-mobile app take up so much space?

Ve a Configuraciones.

Go to Apps & Notifications.

Select See All Apps

Scroll until you find Device Manger or T-Mobile Device Manager (whatever it’s called) and select it

Hit the 3 dots in the right corner

Select Uninstall Updates

Select OK to Replace the App with the factory version

I have no idea what T-Mobile was tracking, but rolling back to an earlier Android version of the app cleared out all the user data and hasn't caused a problem since. Maybe T-Mobile was  taking snapshots or something to make restoring the phone easier. Whatever it was, it should have never been implemented this way.