Accompanied by the sound of a chisel on slate Vamp, managed to produce the following words of wisdom
S'to do with rolling radii and weight distribution. If there's not much weight on a tyre then having wider tyres won't help.
My Cosworth had 225/35/17s on the front and 245/35/17s at the back.
The 911 had, iirc, 205/55/16s on the front and 225/50/16s on the back.