IMO it all boils down to our Interstate highway system and all the freeways around cities. Traffic on these roads typically moves at 75mph or so, and so most folks seem to want bikes or scooters capable of sustaining those speeds. That pretty much sets about 400cc as the absolute bottom limit, and bigger is better.
Most bikes/scooters in the 125-250 cc range can keep up with the slower speeds on 2-lane roads and in town, so if that's all you'll do why go bigger? The smaller bikes are more nimble and economical to buy and maintain, too.
I find traveling 4-lane roads boring, although they are an efficient way to cover long distances. If I've got to use them I'd just as soon take a car.
But if you're out to see the sights and get a feel for the country rather just making time, the 2-lanes and back roads are a lot more relaxed and fun to travel.