They said a lot of things I've been saying all along, particularly that rounded corners and external pockets cost overall volume. One thing they didn't mention is how tapered bags, ones that are thicker at the bottom than at the top, reduce available volume. And wheeled bags not only detract from the volume, they add to the weight.
But then, since the volume that I carry is only about 1400 ci, and regulation volume is around 2700 ci, why do I worry about maximizing the bags volume?
Another thing they don't mention, bags with thicker, heavier weight materials, supposedly higher quality, also detract from the available volume and weight. If you truly carry on, you don't need the heavy weight, durable material - you don't have to worry about the baggage gorillas, you protect it by the way you handle it, and lighter is better. I do insist on quality zippers, but really, the biggest zipper hazard is over-stuffed bags, and I don't do that.