Seems like a lot of people are confusing fuel economy and emissions. Just because a vehicle burns more fuel does not mean it puts out more pollutants...that's pretty obvious. A clean, large engine may well pollute less than a small engine if you look at solely what comes out the tailpipe over an hour of operation. Now, what is exhausted upstream in the process of making the additional fuel the large engine burns, I cannot speak to because I do not know the details.
Doubt it? Here's a little anecdotal evidence (which I do not suggest, but have witnessed- door open)- run a chainsaw in a closed garage for 10 minutes and take a look around. Then (after you clear it out) run a truck and take a look around- you will be able to.
Obviously, a small two stroke burns less fuel than a large engine for a given mile, but that is not the sole measure of environmental impact. In the bad old days before the fall of communism, the most "popular" car in several eastern european countries had small 2 stroke engines that were relatively fuel efficient compared to the more powerful engines in use here at the time. I would sooner have breathed in Chicago than East Berlin any day.
Now does that mean that everyone should ditch their 2 strokes? I am not saying that. They are a small percentage of what is out there that they likely have no measureable environmental impact over here. What the government is trying to do (not that's I am one to defend government logic) is to prevent growth of relatively "dirty" engine....and a little protectionism thrown in- remember, it's and import ban, not a domestic manufacture or use ban.
Where is the balance between mileage and tailpipe producing the optimum environmental benefit? darned if I know.
BTW- it's not just the absence of a exhaust valve- the burning oil may have something to do with it...and it is not double the power, more yes, double no.