A lot can depend on how the baby is laying as well. I was bigger with my DD than with my DS, and this pregnancy I expected to be bigger, but I've stayed smaller than with the other two. It took me quite a while to pop this pregnancy as well. I think the bub was laying quite low in my pelvis and therefore not much of my uterus popped up above my pelvis for longer than the other pregnancies.

Oh, and have you ever seen that show "I didn't know I was pregnant"? I saw two recently where a doctor guestimated gestation as quite early, and then days later the woman were giving birth to babies almost at term! So I think dating a pregnancy or even determining the health of a pregnancy by palpation and size of the uterus/belly can be quite subjective and often wrong.