The due date you are given isn't an exact science. When you go to your next sono they will measure baby and look at organ development that will let you know where baby is and if she is big or your just further along then they thought.
I'm measuring big too but my doctor said he wasn't concerned and that I shouldn't be either.
Was it just using the tape measure? It can mean all sorts i think. I measured a little big most if my pregnancy then it became the exact number of weeks. It can mean youre carrying extra fluid too. Or maybe the baby is just growing really well!