You missed phill's point. The only thing changing is your aspect ratio. The mouse *naturally* moves more to the left and right, because the only thing changing is how your pc is displaying pixels on the screen. The mouse still moves at a 1:1 aspect ratio horizontal/vertical. If you feel like its taking more effort to move left/right, that's just your perception getting warped.
As people have said, this can be changed in your graphics card options.