QScreen::geometry() and QScreen::size() return a scaled screen size, as documented at https://doc.qt.io/qt-6/highdpi.html#device-independent-screen-geometry

You can get the unscaled resolution by multiplying the reported size by the device pixel ratio:

auto scr = app.primaryScreen();
qDebug() << scr->size() * scr->devicePixelRatio();

Suggestion

Provide a convenience function to return the unscaled resolution (QScreen::unscaledSize() ?)