Cass i wouldn't worry about the size of your bump and when they measure you. I am now measuring spot on for dates but last week it was 2wks ahead. Like Kathryn said it depends on which way the baby is and also how much fluid.
I find also it depends on who does it as well as they all touch a different point.

As for size of the baby that i really wouldn't worry about both of mine was bron below 6lb and are healthy and this baby might not even make it 6lb's.