I almost always pick heavy and I fully support the walking turret idea. It just depends on your playing style like everyone else said.
I will always go for the tank character given the choice because that's how I like to play.
My boyfriend always picks the sniper type characters, so it's no surprise he chooses light and svcks it up as a heavy. Likewise, I am not so great as a light.
So just chill, and play as a medium or light since you obviously do not excel as a heavy.