How to delete a line from a txt file after parsing it

kvinvisibleguy Posts 1 Registration date Thursday November 15, 2012 Status Member Last seen November 15, 2012 - Nov 15, 2012 at 09:58 PM
i have a bat file which creates a new txt file to del a line from old txt file and renames the new to old.but the apparent size of the old file shows increase after del process.why?is there any coding which avoids this.