Anyone with Pomme Petite sizing experience?

[perkyderky]

Zulily has a sale and they have a few really cute Pomme Petite rompers.  But according to their sizing chart, my 3 month old should be wearing a 9 month size because of his weight (he's 17.5 lbs).  Does anyone have any experience with this brand to let me know how it REALLY sizes?  I read somewhere else that they do run small, but right now, he's wearing the 3-6 month in Carters and most other brands.  If I wanted to buy something for now, I'm struggling to decide if I should do the 6 month (that's the size I have in my cart right now) or go up even more and do the 9 mos?  TIA