I don't think that is sweat/grease. My crystal case also have this, but since I bought the the ndsl and the case already have this problem from day one. I also clean and dry both surfaces like mad  . I believe it's the material of the crystal case touching the high gloss of the ndsl that causes it. You insert something in between like your own design or cut imagery from magazines etc. Mine has a 'nintendo ds' logo insert.

does the crystal case make your DS significantly bigger, or do you hardly notice it at all?

i'm all for protecting my gadgetry from scratches but i'd rather not have something bulging from my pants all the time.
...well, not the ds anyway.