I don't understand the bigger is better mentality that pervades this subject.
I thinkit is much more than simply bigger is better and I don't think I could hit on all the variables that seem to drive those decisions, but I don't doubt that is part of it. But I think a large portion of it is that so many, many owners are casual owners in that they use the car for a simple pleasure vehicle with some improved capability, therefore they do not do the research to fully understand capabilities and options other than typing on a web board about "what can I fit?" types of questions.
A good example of this is all the various threads we all see on various boards about how I want to make my car handle more like a modern car and will this $5000 suspension do that, when in reality all they really need is a $50 alignment.
So, IMO, a lot of the 17" and 18" wheel migration is because of the proliferation of cheap tires and retro styles that are now available in those sizes, which means the average, "casual" owner can step up to some improved grip with a good look for very little brain power and credit card damage.
There is also the migration towards bigger and brigger brake rotors as well, which necessitate bigger wheels to accomodate them. Now, does everyone who buys a 13 or 14 inch brake assembly fully utilize its capability, I doubt it, but if it makes them feel better having them, then buy away.