I ride a Swift as my main bike, which is about as stiff a ride as it gets on a folder. But even that bike has a rougher ride and more *ahem* responsive handling than a 700c bike. Mind you, this is not to say that "small-wheeled bikes suck," only that there are reasons why larger wheels became the standard in most of the world.

Since I can't read Japanese, I can't explain why they're popular over there.