I've never actually been to Amsterdam so can't really compare with London. I'm not sure the absolute size of the city is really a factor, but the main problem we have here in London is the narrow streets. And a lot of traffic gets funnelled through them, as there aren't many major routes. Thinking mainly of the centre of town... I'm not sure private cars are a real problem, the majority of the traffic seems to be buses, deliveries, construction, taxis. It's not simply going to disappear. The congestion charge was supposed to encourage offset this, but instead I think businesses are just absorbing the cost.