Vision Pro Volume Scaling Woes

I'm trying to get started working with volumes for the Vision Pro. Making use of the tutorials and provided assets. But in simulator, on device and in the XCode preview the volumes are always strangely huge, like double what is required in each dimension. Even the initial project template is like this, looking quite different from what is in the tutorial videos. There is also a full panel backing the volume, where the tutorial suggests this code should produce a background just behind the buttons.

Aside from changing the sphere to a cube and adding .previewLayout(.sizeThatFits) as per the tutorial, this is the template:

Did I miss something crucial here? I want/expect the volume to be roughly the size of the bounding box of the cube, plus a little for the button.

(usually a Unity dev, newb to Swift.)