How To Determine Fetal Growth Through HCG Level Test?

Hi, I am pregnant with twin pregnancy - Dichorionic diamniotic (DCDA) when they first did an ultrasound, my sac size was larger and measured me at 9 wks 3 days, there was no baby inside teh sac so they told me my baby wasnt growing. then i got another scan and they found two sacs and figured my type of twin, also found a baby measure 5weks 3 days. then my bloods dropped from 17656 to 15650. they tried to make me have a currette and an abortion pill. but i wouldnt do it cause i just knew that i wasnt going to miscarry,so i come to a new doctor and got my hcg level tested 3 days later and they come back as 17650s, so now he says my babies are growing at the right rate. My question is : is it possible to my scan is bigger because it has twins ?