Disagree...

I believe the sky box sends a WSS signal via line 23 of the TV picture which tells the TV that the image is a anamorphic image that needs to be stretched width wise. (http://en.wikipedia.org/wiki/Widescreen_signaling) This signal is clearly being sent from the sky box...

With the STB set to output 16:9 and the TV set to handle WS input then the TV picture for TV 1 (broadcast WS content) fulls the full screen.
With no change to the TV, but change the STB to either 4:3 letterbox or 4:3 centrecut then the image is reduced (with black bars top/bottom and L/R for letterbox) or just blackbars L&R for centrecut.
If the STB is set to output 16:9 and the TV told to not do anything with WS signals then the image is compressed.

The TV handles the content correct straight from the Sky STB... it switches to "Full" mode when it gets sent WS content and returns to "Normal" mode for no WS content... this is what I would also expect to happen with images through the Tivo.