The bottle is a little smaller than a 15 lb bottle, of course the 15 lb's refers to the weight of the bottle itself, it is closer to 50 lbs if it is fully charged with carbon dioxide (which we have no way to know whether it it was charged or what it was (once?) charged with).
Well, memory may be playing tricks on me, but from what I remember from my fire fighting training in the Marine Corps, 15 pounds was the weight of the agent. A 50 pound bottle would really defeat the purpose of a portable fire extinguisher. There are many people who would have trouble holding the bottle in one had and pointing the nozzle with the other. Even a 35 pound (25 pounds agent, 10 pounds bottle) bottle is hard to manage at times. Depending on the construction of the container you could add another 5 to 10 pounds for the bottle etc.