I am in my third trimester pregnancy. I just feel my bump doesn't as big as other preggers but doctor said it is ok. But still, i'm litle afraid, is this normal?

Different people have different ways of putting on weight during pregnancy. Partially due to the parents genes too. If parents belong to the small built, baby bump will be small too compared to other mummies to be. Importantly is baby must be gaining weight well and as long as doctor say baby is all well. You do not have to worry! Good thing is that you will be able to go back to per-pregnancy size easier!