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.
For most of the local smaller shows I've been to the Light Heavies typically have the best combination of conditioning and size. heavy and super heavy tend to come in out of shape, and middle weight / light weight unless extremely shredded seem to get over powered... all opinion based