That’s a specific setting, under Settings > Dock & Menu Bar > Double-click a window’s title bar to zoom | minimize. It should default to zoom on a fresh install.
Excellent, thanks — that's very helpful! It's not quite perfect — it does the old 'make it big, but not as big as possible' behaviour that macOS used to do, but that seems to be app-specific, so it actually does what I want for some things.
