When the default QIcon::fromTheme are used with menu items, e.g 
action->setIcon(QIcon::fromTheme("document-save"));
The size is larger than Apple's version on macOS Tahoe (which is now emphasizing menu icons after years of ignoring them).

Look for example at the Save icon with Apple's TextEdit vs. an app with Qt 6.9.1. It's hard for me to measure pixels, but it seems as if Apple's version is at best 24x24 pixels, while the icons used by Qt are larger.

The icons also seem different from SF Pro, but that's a different bug report (139964)