Willow,
I don't think a woman's weight matters that much. When I first got pregnant, I too was rail thin, yet I didn't show (even wore some of my street clothes) until I was almost five months along. One of my (skinny) friends was pregnant at the same time..... I actually gave birth a week before she did. I remember being so jealous because she was wearing maternity clothes at three months and I had so little to show for mine.
A year after our own wedding (1960s), my close (full figured) cousin got married. The style of her lovely wedding dress showed off her small waist and curvy figure. Five months later she raised the proverbial eyebrows by giving birth to a 9 lb baby boy! My guess is that some women just seem to carry more discreetly than others, especially if it's their first time.