I wouldn't breed an '03 that was only 1000 to 1200 grams, unless she was setting a new record for shortness, and had an excellent and reliable appetite. There would have to be some really pressing reason I needed a clutch from her, too.
I would probably not want to hang on to an abnormally small female in the long run. I'm sure most folks meant normal-sized snakes, though, not ones that are exceptionally short or long.
I think it would be too easy for a newbie to convince themselves that their snake is just short, so it's ok to breed at 1000 grams instead of waiting another year. That's the danger in advising folks to breed at lower weights. Someone with a lot of experience can probably tell whether a snake's weight and size are appropriate for breeding at its age.
It's hard to picture an adult BP so small that it never reaches 1500 grams. I'd almost call that a form of dwarfism.