I hardly looked pregnant till near the very end with my son. I remember working out when I was approx 7-8 months along and telling a girl that I was due soon and she could hardly believe it. Fundal height was fine the whole time - he was 5 lbs 11 oz full term. With my daughter I definitely got bigger and even got a stretch mark (none w/ my son) yet my fundal height was 6 weeks behind by the end. But she was born full term and 7.5 lbs.
I don't think it has anything to do w/ CF - just being a woman and your body type. Check out the belly pictures thread.