There are in fact a at least 2 species of poisonous snakes. Both of which are the result of retained toxins from animals consumed in their diet. (http://en.wikipedia.org/wiki/Rhabdop...ng_.26_Defense) The other is poisonous species is a locality of garter snakes in oregon, who retain toxins in their liver from newts they consume. This makes them poisonous to other predators that might eat them.
And yes... a large puff adder can weigh 13lbs. They're very heavy-bodied stout snakes.