I have a feeling you are converting from sub-d back to polygon and retaining the refined geometry level. The effect is that you are baking in the subd refinement into your polygon geomemtry. Sort of like smoothing the polygon geometry setting divisions above 0 and then deleting your history. Once you do this you cannot ever get back the low lovel geometry.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675