Hi Charles, Good question - it has to do with the construction of the head. For example, if we are talking about the AP1 irons vs the DCI irons; although they are both made of the same 431 stainless, the DCI's are a one piece construction perimeter weighting vs the multi-material thin face, perimeter weighting of the AP1's. Our technology has enabled us to increase the forgiveness, give it more feel and responsiveness without increasing the size. If there is a trade off, it's that the head materials that give the AP1's their great feel may look like they wear more rapidly although their playability is not affected.