It is not a matter of the quantity of traffic, but rather "quality" of traffic.
If G determines that most of your visitors are not likely to purchase the products and services offered by advertisers, you can get tossed into Smart Pricing. AdWords conversion rates apparently are a major factor, so if you have a lot of clickers but few people placing orders on advertisers sites, that is one of the major triggers for Smart Pricing.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin