Extreme Tux Racer starts by default with a resolution width of 800 pixels.
Exiting the game should restore your original resolution, but unfortunately it seems the game is crashing when you close, before it can re-resize your display

There is a workaround. You can set a custom game resolution by editing lines in ~/.etracer/options:

set x_resolution X

set y_resolution Y

where X and Y are your desired game width and height. If you set them to your normal resolution dimensions, the game should never "enlarge" your screen to begin with.