I think there's lots of guess work in size guesstimation- my obs called DS at 6-7lb several times (and at 32 weeks he was saying "around 6- healthy but a bit smaller than average") and DS came out (the usual exit) at exactly 8lb. Incidentally, for boys 8lb is right on 50th centile, so actually not that big! In my case my fundal height was always a bit less than gestational weeks so that's where I'm guessing the discrepancy came from. This time round it's the same (and now obs is saying 7-8lb, whatever that means), so I suspect my uterus just has room to expand out rather than up making my fundal height a bit less.

I recall reading about research analysing methods like CT scans and other "accurate" measurement that predicted baby size and pelvis size, and they were remarkably inaccurate in predicting anything- during labour there isn't any way to tell how much the pelvis can stretch, and a lot depends on how the baby is positioned.

I would just let trust in nature taking its course and be confident that if things aren't going right your obs will intervene appropriately.