Motorcycling is much more dangerous than driving. The thing is, almost anyone can drive a car without getting into too much trouble if they are careful. That is not enough for handling a motorcycle.
In a few years I put over 40,000 miles on my bike and I can tell you that it not only requires your constant attention, but even that is not enough for many people. It really is an aptitude thing. Many people simply should never own or ride a motorcycle, no matter how careful they are. They are quite literally an accident waiting to happen.
The size thing matters because a motorcycle can pass between cars if necessary. It can also easily use the shoulder for an escape route when it would be impractical or impossible with a car. Motorcycles also compress following distance when following other motorcycles. Without riding side by side, one can take the left rut and the other take the right. Although one is following the other technically, they are in separate "motorcycle lanes" within a single car lane.
Also, I am able to exploit gaps in traffic that even a tiny hybrid could not navigate without hitting something. I do it several times per commute.
I am aware of the necessity for awareness when riding, as I have been riding since I was 9 (starting on dirtbikes, obviously). I have owned 5 different road bikes, the last being a zx-11. The only two "accidents" I ever had both occured under 5mph, with only me on the bike, and involved the bike simply going over (once because of a patch of soft dirt on a back road, and once due to a puddle of oil in a parking lot). I am mostly looking for ammunition to help educate my wife that motorcycles aren't the guaranteed death warrant she has come to see them as. I am not entirely sure how she came to that conclusion, since I had a bike when we met, and she actually enjoyed going on rides, but even if it doesn't result in my being able to get another bike, I'd like to help her change her attitude.