This is very normal. For most women the more children you have the quicker it will show. Your muscles are still nice and stretched from the previous pregnancies and knows what to do and is all ready to expand.
I have a dear neighbor that had 4 children, with each pregnancy she seemd to be further along earlier and earlier. She always weighed about the same (the last was the least), but when we went back and looked through photos through each pregnancy it was pretty amazing the difference from the first to the last. Her last child was the smallest in weight and she did not gain as much as the first pregnancy, she just carried really large.