I have two girls that grew like weeds. They were 18 months old and ~1400g when they started breeding this season - now pushing 1700g. If they had grown any slower, they would not have been bred this year.
I bred a female last year that ws only 1200g when she started, but was 4 years old. She's over 1600g now and looks like she's going to go for me again this year.
In short, it's a combination of age and weight. Smaller is okay if they're older, but size is a bigger indicator if they're younger (although I still won't push her under 18mos).