It has more to do with sexual maturity (age) than weight in my opinion. You can breed a female in 2 years, but does that mean she'll actually produce? Maybe, maybe not. It's always just safer to wait 3 years.

You can have a older small female and breed her as long as she's in good health and proportionally fit. She can be short, but well muscled and still breed fine.

The weight to length ratio of a snake is just important than the actual weight of a snake too. A snake can be over 1500 grams but be long and thin. I probably wouldn't feel comfortable breeding that type of snake until she gains more mass. Plus a long thin snake doesn't have the same reserves as a more massive snake and would probably make a poor breeder.