I haven't been following the puffer discussions, but would now like to buy a coat for my mom. She is petite, 5'1" with short arms and rectangularly apple-shaped. She normally wears a 6-8 or PM at Macy's and such. However, she does like to layer thick sweaters (never jackets/blazers) under her coats. Should I order the Petite Small or the Petite Medium for her?