I'm 5 months today but small bump

I am a first time mom to be. I am 5 months today and I noticed my bump is small. It's not really showing when I wear loose shirt but when I wear fitted clothes my small tummy shows but it looks like a full tummy after eating meals. My OB told me that I have a small tummy it might be because I'm petite. I'm 5'2 and 44kg. My OB told me my baby is ok during an ultrasound. I get a little jealous when I see pregnant women during my follow up check ups who have a big tummy and the same months as me. I don't look pregnant :(