X-pole jackets: Do they size small or big?

I'd like to order a women's jacket from X-pole, but am unsure what size to get.  As probably many pole-ers know, pole keeps the torso small while making the arms and shoulders big!  I have trouble with many-a jacket, having to switch between size S and M. If I'm a short small girl with developed arms, would you recommend getting the X-pole jacket in a S or M?  Thank you!