Small Gestational Sac

Today we had our second ultrasound. The baby's heart rate was strong (186 bpm) and the little peanut was moving around like crazy. Our doctor was a little concerned with the size of the gestational sac. He wants to monitor the size. We go back in two weeks. I am so excited about the strong heartbeat, but really nervous about the small gestational sac. Has anyone else had this?