fullscreen increases contrast

before after
please help

 

I don't see a contrast difference between your screenshots, so I suppose the difference is in your video driver control panel, you could look at color space, maybe it's 16-255 in windowed mode and 0-255 in full screen, or the opposite...

 

if you look close on phone its visible

 

And then ? If it's that subtle then it's not really a problem, even then have you looked at what @ChatCureuil told you to do ?

 

I look at both images in Gimp histogram, and except that they have a slight different point of view, the contrast looks the same.

To really compare, give 2 images with exactly the same POV.

I'm sure it's something about your screen or driver configuration.