With my first pregnancy I was much smaller than most people(i was 6 months, and ppl thought I was like 3-4 months) ;-) it always depends on your body type,how and if your uterus is tilted,how the baby lies, how much water you have etc. if the doctor doesn't worry, and the baby is fine,i would definitely not worry either:-) it does get more complicated and uncomfortable when the tummy is getting huge...enjoy the "still
not that big tummy" ;-) all the best to you.
I was told that wen I was 24+4 weeks I was measuring 22 weeks so i had a scan and baby was right size , the external measuring isnt a exact science and it can depend on how baby is lying too , if baby is transverse (sideways) u will measure smaller