No - the fees do not appear to based on how much or how little you mined.  They seem to be based on what amount it necessary for a fast transaction.
So far that has been approximately 0.000047 - 0.000054 btc.  So if you are mining a small amount of btc each day, you will have a higher fee cost as a percentage of your mining.  Essentially, if you are mining less than 0.01 per day consider leaving your rewards on the site and withdrawing once a week or every few days.  You will still come out ahead compared to other pools.