I don't know, I find them fairly true to size. I wear an XS in their base layers but a S in jackets. I only have one pair of pants from them -- the fleece-lined ones nylon ones -- and those ran HUUUGE. I ordered a small and had to exchange for XS, but still am grateful that they have a drawstring in the waist. T-shirts are a toss-up, S and XS both work. Jeans are cut very straight and don't fit me unless I wanted to go through the trouble of taking in the waist.
I usually go by the size chart to determine whether an item will work for me -- the measurements they provide have been quite accurate IME.
