Personally, I have bred smaller females. The smallest I ever bred was just around 1350g, when I began pairing her up. She is quite short so she wasn't skinny. She was 3 years old. She gave me 5 good eggs. A 2 year old female I bred at 1500g gave me 4.
Age has a lot to do with it, I think. Not so much size. Breeding females is a case by case basis. If you look at a snake and she's 2 years old, 1500g but looks a little on the skinny side (and this can be because she's long and not really plump), you probably want to beef her up a bit before breeding her. Or you could have a 4 year old female who weighs 1300g but she looks plump and is feeding well. I'd probably give her a chance and start pairing because more than likely she'll hit 1500 or more by the time she ovulates.