Canada is just as full of greasy, nutrtionless, processed-to-hell food as America is, but we simply do not consume as much of it, from my observations.
When I moved to the USA I immediately (over 4 months) gained a buttload of weight (both figuratively and literally...). This is due to my bad habit of finishing everything on my plate. I've since fixed this and my weight is rapidly decreasing. Yay. Serving sizes in the USA are simply ludicrous and completely out of step with reality.
Before talking about high-fiber, low-fat diets, Americans need to do one simple (but difficult) thing: eat less. Eat way less.
A friend and I (both Canadian) paid a visit to the Cheesecake Factory this last weekend - when our plates landed on our table our eyes bugged out. There was enough food to feed 4 people in a standard Canadian restaurant. I'm not even exaggerating :S The table next to ours polished their food single-handedly and then ordered dessert. We were shocked. How any single person can consume that much food in one sitting is unimaginable, and somewhat disgusting.