Yeah, like a Twingo can pollute more than a Cayenne, depending on how you drive it, but a small car which has a low consumption will generate less dirty things than a big one with high consumption. Even if the new ones are cleaner, there are some things that you can't "clean" and the atoms contained in petrol must go out in a way or another.

I wonder if there are really so many pre-1992 cars in Amsterdam I wonder if that would make really a big difference.

It is probably politically impossible to ban SUVs or other cars with high consumptions, but easy to forbid access to old cars.