What exactly it is has been the subject of a few heated threads... But there is something called a 'pygmy stimsons' which is nothing to do with perthensis. Yep, Snakeranch has some adults for sale ATM.
Thes were being sold at least as far back as 06. I too would like to know whether they are just a morph or actually are crossbreed. Obviously they never really took of, if they are a hybrid I really don't know why people would bother. All the pictures I've found are pretty ordinary.
Pygmy 'stimsons' are basically a natural intergrade between spotted pythons and stimsons pythons, they come from north Qld. The pygmy python (A.perthensis) comes from Western Australia and is a differant species. Both are available from the snake ranch and various other breeders.
No its not that simple, its likely that they have just evolved to be smaller over a long period of time. While they are short they are still very fat like a spotted python if not fatter. I have no experience in the area they come from so trying to suggest why being smaller was an advantage would be pretty much a random guess. That said being small or big has various advantages and disavantages that come with it, considerable size differances are fairly common in many species of reptiles.