I noticed that my DNG files grew to massive sizes when making numerous brush stroke changes to a photograph. One DNG went from 30MB to 425 MB. After updating the DNG and metatdata, the file shrunk to a normal size. But, I notice that the photograph is slow to load ever since then. I thought autowrite to xmp files, when checked, updates the metadata. If not, when does it help? does is it help when exporting to Photomatix?