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

Are you sure it isn't saving things, like your bills on the device?  That seems like a lot of space for one app to use unless it's storing something. 

ADB is your friend!  If you really want the app gone.  Look up an ADB tutorial on XDA.  It is fairly easy, doesn't require root, and can be very useful to remove unwanted apps and bloatware.