I think a lot of it has to go by what we see and know about our snakes. A male at 1000 grams might look bulkier and BE more overweight than another male at 2000 grams. I remember just earlier someone posting about a ball python over five feet who turned out to be a male--I would bet that that male's 'healthy weight' is far more than my male who barely managed to hit three feet no matter what he was fed. The first year we got him he was 500 grams--and he was almost nearing the seeing skin between the scales pudginess! He grew a little length, but he's just a shorter snake(wild caught, possibly didn't get any good food when he was young, possibly had two short parents and genetics are messing with him, or maybe he's just a smaller snake. Maybe he got the 'short straw')
I know Maru at anything over 1000 grams would be a fat fat fat snake, but I know others would need a bit more weight on their bones!
Know your snake and watch for telltale signs and issues! I see people near me with pet ball pythons that they want to be huge--they got a bp because they thought ALL bps would get to six feet. So they feed it to get it to be that size. Some of them got to grow huge..Others are more like Maru, and once they get to a certain length, just seem to hit a wall--and then they start 'growing' wider instead of longer!
I think it's important not to over or under feed ball pythons, adults or babies. It's amazing, but I have seen obese babies before.And I'm not talking about egg yolk tummies either! Know your snake and what it should look and act like! Find the balance. Ball python males shouldn't look like a fat blood python, but it shouldn't look like a skinny corn snake either. Not that anyone here has done that to their snakes.
I think the 1000 gram 'rule' is a good guide to give people an idea of what they should keep an eye on, but we should remember not all animals are perfect or like each other, and be ready to make changes for each individual.