I look at a combination of things when considering a snake for breeding.
1) body condition
2) age-maturity
3) size.
Most people just blindly breed their females at 1500g.
But it's not just a set weight. You have to look at the snake's overall body condition and maturity too. A mature short chunky female at 1200 grams is a better breeding candidate than a long skinny 1800 gram girl, for example.
That 1200 gram girl has a better shot at enduring the breeding season with the better body condition and fat reserves than the skinny 1800 gram one. And some girls just never get large even though they can be a few years old.