If this fat guy could get great deals on clothes, imagine what the average joe could find.
High volume sizes sell out so they don't typically hit the secondary resellers like Sierra Trading Post with as much surplus merchandise as the really smalls and really bigs. Medium and Large shirts sell out quickly, if they were even in stock in most cases. XXL and Small will almost always be available at huge discounts above what Medium and Large sold for at the same retailer before they sold out.