In my experience (so far) it appears to be a combination of the two.
Some animals will grow significantly faster on the same amount of food.
Without a doubt, feeding them more also makes them grow faster. I have never seen a truly obese juvenile ball python. They simply don't get fat, they grow instead.
I have an '08 pastel that is now over 1200 grams. I fully expect her to make 1500 grams for me this year, and she'll be just 2 years old. I have another '08 that is only 800 grams. They were on the same feeding schedule, so go figure.
I've also heard of folks getting animals up over 1000 grams in the FIRST YEAR.
They aren't obese--they're bulky, but long. They've simply grown that fast.
The first few years are very important for a ball python's growth--I doubt any female underfed in the first few years will ever reach a significant size. Anyone who has seen otherwise, feel free to correct me.
This would be at odds with the 'slow grown' trend I saw a year or two back, where folks would advertise that adult females of age 4 or 5 had been fed lightly so they would grow up more slowly...with the idea that they would be healthier. There doesn't seem to be anything to support that notion, though.