Touring Equipment

my audax bike has 700x28 36spoke wheels, used for S24O's with just a saddlebag and bits strapped/stuffed onto rack/back pockets. 700x35 36h wheels loaded with carradice super C's + bar bag for 2weekers.

What sort of volume are the Super Cs and how do you find that kind of space? Is the larger tyre size for taking the extra weight or to allow for unpredictable road/trail surfaces? Cheers, I'm learning stacks here.