I had a girl a few years ago that weighed 1350 grams the day BEFORE she laid, and gave us 9 good eggs. After laying, she weighed right around 900 grams, maybe a little less. All 9 eggs went full term, all 9 eggs hatched, and all of the babies were healthy, and started eating as quickly as any of our other clutches. She Got right back on her feeding schedule, but she's just a smaller female. The next year, she was probably only 1300 grams or so when she bred, and still gave us a clutch of 7 or 8. BUT, the key here is, she wasn't 18 months old. She was 4 or 5 years old, but just happened to be on the smaller side of things.
On the flip side, I had 2 girls this past year that were both over 1800 grams, but were only 2 years old, and neither of them went for us. Oh well! Now they're over 2500 grams, and 3 years old, so hopefully each should provide a nice healthy clutch for us. I firmly believe age is much more important than size when it comes to reproduction (at least in my experience)