Also, your stomach muscles are looser as a result of already going through a pregnancy and won't resist (hold your new bump in) the same way they did the first time around.
I personally believe it depends on your frame, uterus position, etc. Your uterus actually does grow a lot in the early weeks and if my memory serves me correctly, it approximately the size of a grapefruit by week 9!
I definitely popped early with this one (my second) and think it's totally normal. Some of it is water retention and bloating from the progesterone slowing digestion (and often constipating you too!) and some of it is due to the uterus growing.
You always show earlier with your second, third..... You just may be bloated the because the baby isn't big enough yet to start showing.
I'm 5 weeks with my 2nd and I've noticed a lot more bloating this time around. I'm heavy to start with and it just makes me feel even worse.
This is my second and I'm 11 weeks - I've been wearing looser clothes already, but I do think it's more bloating than baby for me (I'm heavier to start with). I've actually gained less weight this time than last at this point but feel fatter.
More than likely it is bloating and/or water retention. It should start to go down in the coming weeks.