Is baby small?

Hi I had my 20w scan 3 days ago and the sonographer said everything was fine measurements were OK. But I was reading my app which tells me how Big Baby should be and what she looks like etc etc- it states baby should be 25cm this week but I measured out at 15.2cm- before the scan my consultant says they will be giving me more growth scans as my previous child was small at birth. (6lbs 6oz) but back then they never said anything when he was born? Consultants said if she is small they will have to deliver early to stabilise her :( I shouldn't worry as I'm only 20+5 and my next scan isn't until week 30 but I can't help but worry lol has anyone had similar? Xx