On the state and local level, the middleweight class is typically the biggest. That, or the light heavyweights. At that level there aren't a ton of guys that have added enough muscle to be in the heavier classes (unless they are just out of shape).
When you move up to the Jr National level, those two are usually the biggest classes as well.
At pro-qualifiers, almost every class is very big. At last year's nationals I think every class had at least 30 competitors.