When buying apparel items online always be sure to rely on the actual measurements in the item's listing vs. relying on the label size only as many manufacturers surprisingly have different measurements for what they, for example consider a large, extra large, etc. Not to mention (see above), washing, drying, etc. can possibly alter the size of a garment over time. 

Prior to purchase, please do take a  medium weight vest (if you don't have one, you could do it with a jacket), flat lay it on a bed or floor, measure it and match measurements with what you see below to see if it will properly fit.