The Quantum is on the thick side because it’s made with two layers of nylon webbing, tacked together about every 3″ (it varies) to create a nylon ladder; to fix the leash around your waist/shoulder/an object, you clip into the ladder with the included carabiner.
The Small fits short in the back but looks fine- according to the product description, it’s designed to be this way (although it might fit very short for dogs in the upper end of the girth range). Coverage-wise, this jacket is a little lacking; the armholes are large and it’s cut low in the neck. However, it layers fantastically- Robin’s usually not happy about wearing more than one jacket because of the bulk, but he doesn’t even notice the addition of this one. Overall it’s an athletic, flattering cut.