There is less degrees on the screen (lower degree/pixel), so if you continue to rotate the same degree/count then you will rotate more pixels. You have to reduce the degree/count to rotate the same degree/pixels, and end up with a longer distance to rotate 360 degrees.
Degree/pixels only uses the middle pixels. If you know there is a total of 103 degrees horizontally (Overwatch), which is across 1920 pixels, then you can scale that down to find how many there is in 1 pixel.
2 * atan(1/1920 * tan(103/2 * pi/180)) * 180/pi = ~0.075032
If you drop to 30 degrees vertically (Overwatch Widowmaker), then you end up with less degree/pixel.
2 * atan(1/1080 * tan(30/2 * pi/180)) * 180/pi = ~0.02843
Since the game rotates a fixed amount of degrees/count for each unit of sensitivity, you will want to reduce the sensitivity when you scope in (by ~0.028/0.075) to keep the same pixel ratio. When scoped, you will have to rotate ~38% of the original amount to preserve the sensitivity of the mouse, but you will end up with the inconvenience of a larger distance to turn 360 degrees.
(2 * atan(1/1080 * tan(30/2 * pi/180)) * 180/pi) / (2 * atan(1/1920 * tan(103/2 * pi/180)) * 180/pi) = ~0.378909