Fitness level has more to do with it than anything. Also the term sire pertains to the male in the relationship. I'm a fairly skinny girl but I have a short body which made pregnancies a little hard because the babies were always pushed up against everything. I even cracked a rib with my last pregnancy. My friend who is shorter than I (by 9 in.) and much larger bones with a lot more weight had awesome pregnancies with no morning sickness, no bladder problems and very little heartburn. Now that's just the pregnancy part, in actual labor, if a woman is really overweight she won't have the muscle tone needed to have a quick labor or to push the baby out. There's also other factors involved, like pelvis size which has nothing to do with weight or bone structure. My friend has nicer hips than me but the opening in the middle was too small for birthing and the babies wouldn't fit. I have terrible hips but the opening is large enough that I had awesome, quick labors. So really, in answer to your question, size of the woman does have some affect on the ability to have children, but it's just one thing in a whole lot of other factors.