One tech support guy said that you could fill the screen by taking standard DVD content, then upconverting it to 1920x1080, and then playing that back from USB.
None of my content has been converted from 720x480, so the black bars are very large.
And since you started out with BD content, you didn't have the stretched heads either I guess
In the end, the TV is just a panel in front of a computer running software. I just think that if they wanted to modify the software for it to work right, then it could. At the very least they could give you manual control over screen options via a menu, but there's really nothing there for the USB input.
Thanks for going to the trouble to do a test.