I received my blazer today--LOVE the burgundy color and the peplum is very flattering--and I do think that it runs small. I bought the Small Petite and it JUST fits in the armholes and waist with a sleeveless top underneath. The shoulders are a tiny bit wide but I certainly couldn't size down; if I had more of a bust, I would consider going up to a Small Regular for more arm mobility.
I'm normally a size 2 in structured button-downs and size 0 in knit tops, very small bust and a waistless torso. I think the XSP in this jacket could probably fit a 00 who has not-narrow shoulders. The SP fits me like an Anthro size 2 or a juniors' size XS.
