That size difference on Windows and some reading made me realise the reason for the increased size on macOS: it includes both Intel and ARM architectures.
https://stackoverflow.com/questions/...-so-big-in-qt6
So, it seems that it might make sense to generate separate Intel and ARM builds of Calibre for macOS just like what is already done with the Linux builds?
https://download.calibre-ebook.com/6.15.1/
This would roughly halve the size of the app on macOS.