13+1 question about the size of my baby

Amanda • Happily married for 13 years ❤️ Mommy to 8 beautiful kids

I had an ultrasound Monday (I was 12+6), everything looked good! Baby measured 6.31 cm. The app says at 13 weeks, baby is 7.4 cm. My doc hasn’t changed my due date and this is the third scan I’ve had. She said baby is doing great and measuring correct for how far along I am. So is my baby on the smaller side or the app wrong with measurements?