Dont stress too much. I am pretty much the same as you and my fundal is only 28 cm as well. I had an ultrasound done and the doc said to me after that that it can depend on which way the baby is laying and stuff.

Mind you I am the complete opposite to you, I am 5 foot 9 fit in XL and I have a reeeeaaaalllly long torso. 8-[