I picked up a 6'3 in a surf shop yesturday and it felt really good under my arms as well, it was not chunky feeling and the 6'1 and 6'3 felt very similar. They also had a 6'5 HR and compared to the 6'1 and 6'3 that felt and looked a lot more of a chunkier board.

I have noticed that some sizes of Firewire models seem to have a couple of sizes that dont feel too different from each other and then the next size up just feels massive. The Domainator the 5'6 and 5'8 feel very similar but the 5'10 feels and looks a lot chunkier, picking up a 6'4 Domainato and comparing it to a 6'6 you can physically see the different in volume through the rails and the tail between those boards.