As Rob said, this is normal, specially if you are using ringing protection. The reason is that deringing (and also the wavelets regularization) uses a multiscale algorithm that makes the result dependent to the contents and scale of the image. Nevertheless, most of the time the parameters are close enough to fine tune it quickly using a preview of the full size of the image, after you reached a good set of parameters with the smaller ones (also, try using half a dozen of "small" previews in different parts of the image, to control the result with different structures).
