In what ways is "The Celebrated Jumping Frog of Calaveras County" a humorous story?

One of the ways that Twain sets up the story to be humorous is by making Simon Wheeler into a sort of straight man. The narrator describes him as telling this rather ridiculous tale about Leonidas Smiley and his penchant for betting but never once changing his tone, varying in his enthusiasm for telling the tale, or any other sort of change. He is the very definition of the straight man except perhaps for his rambling and at times almost incoherent style of story telling.

The story that Wheeler tells is funny in that it continues to grow more and more absurd with every animal that the Reverend Smiley had. He had a "fifteen minute nag" that was so slow-looking that it would be given a head start and then find a ridiculous way to come out of nowhere to challenge for the win at the end of the race. He had a bulldog with no hind legs named Andrew Jackson that could out-fight any able bodied dog by out-witting them and then laying hold of them with his mighty jaws. And he had the famous jumping frog that could out-jump any frog around.