Everyone is different, hey? I started getting bigger straight away, feeling bloated etc. Lots of my tighter work pants stopped fitting me around the 12-15week mark. (Course, that could've been due to all the hot chips I was eating at lunchtimes
My friend is very athletic and is currently 21 weeks pg - I can't see a difference in her at all! (Though she can see the difference herself, and she did show me her little 'bump' hidden under her clothes It is little but it is definitely there! Just can't see it when you're not looking for it, IYKWIM.)