Hi,
Using an orthographic camera, it isn't possible to change the zoom by pure QML (without the wheel within a Viewport). We can't use "fieldOfView" with this "projectionType" which is normal, and eye position doesn't change the zoom which is also normal. Moreover "viewSize" is a non scriptable property... There's no way to change the zoom for an orthographic camera, this projection type is unusable in QML.
Actually it's possible to have the wanted behavior scaling items but I don't think that's a good way to do it.