I disagree with this. You have to look at the weight to length ratio of the snake and overall body condition. Not just weight alone.
You can safely breed a 1200 gram female as long as she has good body condition.
Some females are small and just never grow any larger. They can still throw average sized clutches successfully.
Some females who are 1500+ grams are extremely long and skinny, which is not an appropriate breeding body condition in my opinion.
Some 1200 gram females are short and have good fat reserves, which is a better breeding candidate than a long skinny 1900 gram female.
I'm currently breeding a girl under 1500 grams, but shes nice and chunky. She has an extremely good body condition.
I also have an 1800 gram girl, who I will not breed. She is really long, skinny, and hardly has any fat reserves. Not a good body condition for a breeding female imo.