I wasn't able to discover that Designer uses the Command key to center-resize, so I just assumed that the app didn't do this common operation. I don't use any other Affinity apps, so consistency between your other tools isn't a compelling argument at all.
On the Mac, the option-key has been used for this behavior since before Windows existed. (Mac Draw, Illustrator 1.0, etc.)
Heck, you can center-resize every window on the system (including Affinity Designer) by holding Option when dragging.
Not following this platform convention makes Affinity Designer feel broken on the Mac.
