Baby size at anatomy scan

I am plus size, but was a decently small baby (6 lbs and 21in long) but my husband was 11 lbs 9 oz when he was born so we were expecting a big baby. Well at the scan she was 11 oz and in the 31 percentile (at 20w1d). They didn’t really act concerned and said anything under a pound is good at this point. But when the ultrasound tech was measuring her I saw her belly was measuring like 18w4d or something and her legs 19w4d and her head 19w. They didn’t get any heart measurements because she wouldn’t flip (she was facing my back). I have to go back for heart measurements next month. Anybody else experience this?