Both MrsMurk and I wondered "Didn't they used to be bigger?" I find I'm not the only one who thinks this!
There have been many debates amongst fans of the biscuit about the size. Wagon Wheels have supposedly shrunk in size as time has progressed, but Burton's Foods Ltd have denied this. It has been suggested that the supposed shrinkage is due to an adult's fond childhood memory of eating a Wagon Wheel held in a much smaller hand. In Australia however, Arnott's have stated that tray packs of Wagon Wheels were in fact Mini Wagon Wheels and have re-released the original 48g Wagon Wheels.
The website 'a nice cup of tea and a sit down', linked to above, has this to say on the issue:
Wagon wheels create an instant sense of nostalgia, and yearning for days gone by, in all seasoned biscuit fans, due to the fact that they used to be bigger, much bigger, and thicker. The reduction in size of the Wagon wheel maybe due to our childhood memories recalling a biscuit that was relatively larger compared to us. However, this phenomena does not occur with other large diameter biscuits such as the digestive, so we are left to wonder at the reasons for a mysterious plot to reduce their size.
Whatever the truth of the matter, whether Wagon Wheels have indeed shrunk, or people just have faulty memory, there is a marketing opportunity here. They should conduct a survey to discover what people remember about Wagon Wheels (bit gooey centre, broke, but held together by goo, about 2cm wider). Then they should market the 'Wagon Wheel Nostalgia: Just as you remember it' (with smallprint on the back saying 'this isn't how it was, it's how you remember it to be when you had smaller hands).