Low-profile lightweight blaze orange hunting vest made my Orange Aglow. Rear panel has extra material to fit over extra layers. Over 600 square inches of orange on each size. 

Sizing - Orange Aglow recommends ordering your t-shirt (not undershirt) size.  The vest is stretchy and will fit over a few layers, but you may want to order up one size if you will be wearing bulky clothes.  The Adult Small loosely equates to a Women's Medium.