It has a lot to do with your measurements not just how big people perceive your bump to be - if that makes any sense! When the midwife measures you, she feels around for where the top of your womb is and if your baby is on target, big or small for dates. It is also normal to measure one to two weeks out either way as well (as baby can sometimes have sudden growth spurts and then slow up or might just have very tall or small parents).
It can also depend on the way you carry your baby or if you have put on a lot of weight - it does not necessarily mean your baby is going to be massive.
I didn't put on much weight at all (under two stone) and from behind did not look pregnant and my bump, though sticking out the way (I carried up front) quite a lot was still fairly neat. My little boy was nearly 10 pounds!
I wouldn't worry - if there is a concern over the size of your baby, your midwife will inform you and you will go in for a scan to check the baby's size. However, it seems very normal to me.
Good luck!
xxx