I think the 18's don't quite fill the arches as well in the MK7 as they did in the Mk5/6. The arches are definitely bigger in the new car! Years ago, 16 or 17" alloys were considered large, now every 2nd Polo or Fiesta has alloys in that size. Wheel arches are getting bigger and to look anyway decent, a car needs meaty alloys to avoid the castor wheels under a couch look!
Ride quality inevitably suffers though to some degree, but I do like a firm ride.