dnaop-wr

The game is displayed in full screen, but when I take a screenshot, the image is 4:3. Otherwise, the image is displayed in full screen in the game.
Here, I took a picture with my phone:
http://savepic.ru/14098728.jpg

Added 16 minutes later
Roman2211

I found this information:

1.25:1 (5:4)
Early computer monitors with a resolution of 1280x1024 pixels had this aspect ratio. In everyday practice, they are often attributed to a 4:3 aspect ratio, which is not entirely correct. In the 2010s, they were gradually replaced by widescreen monitors with aspect ratios of 16:10 and 16:9.

Apparently, the resolution of 1280x1024 is native to your monitor, so everything works perfectly.