Hi K82, I think it varies from person to person - I was 5ft8 and a 12-14 before I was pregnant and didn't show until I was nearly 7months, then sprouted alot and ended up having a baby weighing nearly 9lb!!! My sister in law is a much smaller frame, probably size 8-10 and only 5ft2, and she has been huge since very early. Another friend of mine was showing from very early, 12 weeks she looked pregnant, but went on to have a 7lb baby.
I think as long as you are well and your midwife is happy then there is nothing to worry about. You may find that you don't seem to grow for weeks, then all of a sudden in th space of one night it has shot out again! I also found it depended how baby was laying as to whether I looked enormous, or just had a delicate bump!