I'm 21 weeks and 3 days.

First pregnancy? If yes, then it's perfectly normal! When you see other mums that shows bigger than yours, maybe it's not their 1st pregnancy. Subsequent pregnancies shows obviously more than the 1st. This is because their uterus has already stretched from the previous pregnancies. Also nothing to worry about your bump size cos it does not determine your baby's size or weight. I will just shrug off and ignore them as long as my baby is healthy. On another note, with smaller bump it is so much easier to slim down during confinement period.