You can neither scale groups nor move them and have physics work consistently.
All groups must have the same scale and coordinates.
Additionally, scaling the display object has no affect on the body.
Read the snippet in the ‘important’ box on this page:
https://docs.coronalabs.com/guide/physics/physicsSetup/index.html#physics.setdrawmode
Set hybrid draw mode to see what I mean.
https://docs.coronalabs.com/api/library/physics/setDrawMode.html