This for us is not first pregnancy but will be first DC and I've apparently got a big bump but only one in there, albeit one who has consistently measured big.
I'm now 25 + 1 and keep getting asked when I'm going to be induced as "it's not good to be that overdue" and my mum keeps telling me my bump is bigger now than hers on the day she had me, and from the photos she looked pretty huge 
. We had a trip to the delivery suite yesterday as the little man stopped moving for 12 hours (was just being awkward) and the student midwife who we've not met before wouldn't believe I was 25 weeks, and kept asking if I meant 35, and then kept asking if I was absolutely sure of my dates, luckilly a familiar midwife came and told her I wasn't lying 
.
My bump started showing early, about 8-9 weeks (def bump not bloat, confirmed by consultant), and by 12 weeks the consultant said I had a bigger bump than most of the ladies who were 20-25 weeks. He seems to find it hilarious 
he very good really though so I can't complain.
DH was convinced it was twins/triplets before first scan, but nope, definitely only the one. I am quite tall and pre-pregnancy was pretty slim and toned (5'9 and a size 10) and all the midvives (complicated pregnancy so getting very familiar with the hospital midwives, although so an not to worry you there's absolutely nothing wrong with the little man, just with me) keep saying how strong my stomach muscles are, so not sure if this has made a difference?
I've also had bad hyperemesis and as a result haven't put on any weight yet, so I think that perhaps makes the bump look bigger in relation to me, could this be the case with you?
Oh, just remembered, I was also told that my uterus sits unusually high in my torso (although was told this by a v. useless sonographer, so not sure if that's right as never heard it before), so perhaps this is a factor too.