Yeah, it's space. Compared to the giant hunk of metal that's soaring through the air, most hams probably weigh relatively little and consume relatively little fuel. It's just when they droop over the arm rest that the problems happen. As for luggage, I think that's because, due to labor/safety rules, the people who load your luggage aren't allowed to lift something that weighs more than 50lbs without special equipment, hence the extra cost. And since even hams usually load themselves on to planes (not always, I know, I know), they don't have to pay for special equipment to be loaded.