When Marquardt was fighting as MW, he said he walked around at 195-200. He was definitely much smaller than guys like Sonnen and A. Silva who walk around at 210-220.
He does not have to lose 15 pounds of muscle - maybe 5-10, but not 15. GSP walks around at 190, only 5-10 pounds less than Nate. He will go from being a smallish MW to a big WW.