If the Windows scaling is set to a value over 100% then Kart Racing Pro will start at a lower resolution to compensate for the automatic scaling.
To solve the issue and use the full resolution: right-click on the Kart Racing Pro icon -> Properties -> Compatibility
Then, on Windows 10:
Click on Change high DPI settings -> "Override high DPI scaling behavior. Scaling performed by:"
Select "Application"
On Windows 7/8 instead:
Click on "Disable display scaling on high DPI settings"