Cabin crew usually man main cabin doors. Over wing exits (of the hatch type) are usually self help and the cabin crew do not operate nor do they supervise evacuations through the over wing exits. So there's nothing to stop someone trying to get through but I should imagine most people of size who May feel like they won't fit will not go near the over wing exits as they wont want to get stuck in a potential burning aircraft.
The size of people is not the issue. Taking cabin bags through over wing exits is what will kill people, not an overweight person.