'big baby'

Azania
Hi ladies, so this is my first baby so I have never ending questions! I have been told I'm measuring perfectly for 28 weeks with my bump measuring 28cm (although I have a pretty small bump as shown below) 
  but have been told she's a "big baby" at 20 weeks she was above average for every measurement. I passed my GTT testing this week so I'm not overly concerned. My boyfriend is 6ft 5 🙈 so I guess this may effect it but was wondering if/how a midwife can tell I'm having a big baby?? Thank you xxx