The major issue with the IIHS traffic statistics, is that they didn't compensate for the age of the drivers. Small cars are cheaper and bought by much younger people than full sized sedans. Young people have roughly 3 times the fatality rate as middle age folks. Coincidently, the difference in fatality rate between small cars and full sized sedans is 35/1M versus 11/1M, pretty much three times.
You forgot to mention all the accident-prone old geezers in your analysis. Last time I checked, they prefer to drive those same full size sedans.
If smaller cars were truly that much less safe, the fatality rates in the US should be much lower than Europe, which isn't the case. The US death rate is higher than in Canada, which has similar training levels, smaller cars on average, while still having a large number of large SUVs and commercial vehicles.
But in Europe, there will presumably be more cases of small cars smashing into other small cars. In the U.S., you've generally got big cars smashing into big cars. In both cases, it's a fair fight.
As for the difference between the U.S. and Canada, the first thing that comes to mind is their 75mph Interstate system. I suspect that our average highway speeds are much lower up here (and I'm talking the whole country, not just one stretch of 8-lane in the GTA). That alone could be a huge factor.
