Very interesting jnjreptiles! Many animals show diversity within the same species based on region. I was speaking with a friend about his Green Tree pythons (he owns 13) and he was explaining the amazing color variations based on locality. For example he was telling me about one of his adults which retains the juvenile yellow coloring and is found in a specific area of new guinea (which I guess makes it a yellow tree python). I have also observed differences among common brown house snakes in different regions of south africa.
The specimen I saw at the museum was also heavily bodied, and if memory serves I saw some very large specimens at the Transvaal snake park in my youth (along with the first Anchieta's dwarf python I had seen, a close relative). So it could well be the locality you mentioned favors larger size in this species.
If you can get a hold of any pictures and post them that would be great, along with any details of age, locality found etc.
Thanks!,
Dave