Yep, most subdivision algorithms make the model smaller, so that it is pretty useless for Poser clothing. It might have some advantage for Poser props like vehicles or such, where it can create smoother surfaces. But usually the model has to be created with subdivision in mind to have a good effect. Poser 5 does subdivision at render time (the infamous polygon smoothing) and for most current Poser props this is not a good option. ;)