As p0diabl0 mentioned they're roughly 26L. They're 26.8L to be exact. Not the biggest but large enough for everything I've wanted to do so far.
The fit to the bike is pretty tight but I haven't seen them next to any other set-up though so I can't offer a comparison. I did measure and the width at the outside of the cases is nearly exactly the same width as the handle bars.
I can also note that I have done some mild off-road riding and have dropped the bike while at a near stand still (momentary lapse in concentration while moving the bike into the garage
) once since my last update. Besides a scuff in one of the cases everything is as tight and solid as the day I installed them.
Having used them an average of somewhere around 3 or 4 days a week since they've been installed for everything from camping to grocery shopping to hauling my laptop and various parts for work I think having the ability to easily remove the cases, pack them up, and then quickly mount them on and remove them from the bike while full is the way to go. On one camping trip the wife and I had to walk a ways from where we could park the bike back to the campsite. We just popped the cases off and carried them back to the site; piece of cake. When packing for a trip we can bring the cases in the house, pack them until they're about to burst, and then mount them on the bike. I have experienced a wide variety of scenarios like these where any system that doesn't allow the cases to be easily taken on and off the racks while the cases are full would have been a pain in the a$$ to say the least.
Overall I'm still very pleased and don't have any buyers remorse.