Now, when you say "full-screen" do you mean playing the file and then hitting the "PrtSc" button?
If so, wrong way to take screenshots, either use MPC-HC or VLC, and then upscale the lower screenshot to the other respective resolution, and then compare.
I upscale using GIMP and Sinc (Lanzcos 3), not too sure how good Paint's upscaling is... not using it anymore..