Fundal Height measurements in theory should be Number of weeks pg = cms in height, but this can have so many variants - like the position of the baby, amount of fluid, your body shape - all of these can affect the measurement from week to week. You might find that baby is sitting quite high, and making the 'measurement' seem large, but once baby engages, this measurement can go down. Also because this is a 2nd pg, the uterus can stretch quicker, because it has done it before.

Many Obs/Doctors don't even measure the fundal height, but it can give them an indication that if your belly is growing, the baby is growing and not to do with size prediction at all.