Baby is measuring small. Should I worry?

I'm 21 weeks 4 days. I had my sonogram today. My appointment went well baby is healthy and moving so much. I'm doing well as well. The tech said baby is measuring at 20 weeks instead of 21 weeks. Baby is 12 oz instead of 12.7-13 oz. She says I shouldn't worry. But I start to feel so down when I hear this because I'm a small woman I carry small. Everyone expects me to be big and gain a whole bunch of weight but, I don't and i get criticized because of how I'm carrying my child each time. I told her my last baby was 6lbs 3 oz she said that small but my midwife says that's not bad. I eat a lot during the day. It makes me feel as if I'm not doing right even when I know I am. This is baby #6 Im very active. I apologize for going on and on I just needed to vent. I have 4 boys (14/11/7/2) 1 girl (5) and in the end I found out ill be expected another's baby girl 💓. I just want to do right by my kids and my new arrival is all.