ok, just measured Minnie and right behind front legs her measurement its 12 1/4 inches.
and the measurement that is biggest part of her chest ( not where you measure ... is 13 inches. )
i do remember when i first ordered , i picked the wrong size also. i got the same size as you the XS/S at first. then put in another order for the XS and it fit perfect.
if you order the XS and they are too big, i will buy them from you cause thats my girls ( peyton and minnies( size) and i love having different colors. right now , they only have one each.