Hmm.. that doesn't quite do what I'm after — make it the full size of the screen. It looks like it's similar to the old macOS behaviour that made the window 'as big as its contents'.

It is exactly that behavior, yes (and that historical oddity is almost certainly why). I wish this was configurable somewhere, and wish the behavior of the green button was also configurable so you could flip it to “maximize with no modifier key, full screen with”.