I think I found the problem:

The images are not really different. Araxis Merge shows the size difference. In Photoshop both images are exactly the same.

The difference is that if I use QCoreApplication the image's dpi value is 75 but when I use QApplication it is set to 72.

Araxis does a scaled compare this is where the difference comes from...

But: I still think that Qt should not set other DPI values when using a QCoreApplication vs QApplication.

Currently I am searching for a way to change the output's dpi value...