When they feel your belly they are feeling for the "fundus" which is the top of your uterus.
They know how much it should grow by certain dates and within parameters they can give approximations on weight.
So you are both technically right.
Like say you were 34 weeks and your fundus was at x measurement then your baby would be average sized but if your fundus was at z measurement then your baby would be looking large for the gestation.
It's all educated guesses.