Out of curiosity what profile 13's are you running?
185/60/13 were the ones i was using on my previous mk1. Good size i think.
If you compare that to the common 195/50/15 tyres most runs on 15's you get the following..
185/60/13
Rolling Radius 276.1mm
Circumference 1734.79mm
Diameter 552.2mm
195/50/15 (*note this is about the same as a 175/70/13 tyre)
Rolling Radius 288mm
Circumference 1809.56mm
Diameter 576mm