Okey cokey - I think I've just realised where I've been getting confused.
Until now I'd assumed that the rolling out of the rear arch was to accommodate a larger diameter wheel but by reading back through other threads I've picked up on the fact that it's the width / offset or spacing of the wheels that change to give the car a wider footprint and look better with the wider bodywork.
Makes perfect sense now!
Daft bugger!
It's amazing how clear things can become at 4am!