I have the Saddleman Large Drifter bag (which look) similar. They are 15" x 10.5" x 5.75. I can just barely get my Triumph Retro Paddock Jacket with liner/pads inside one bag. That size fits most of my casual, non-touring needs. If I went on extended trips with the bike, they'd be too small. You also have to consider the width you are adding, I think too fat looks strange. Than it becomes a form over function thing for the owner.
Those Jim's bags do look like lifetime bags. My Saddlemans are six seasons old and will eventually need to be replaced.
