Windows 2003 Security Log Question

I am running a Windows 2003 server at work and my security log keeps getting full. I have went into the security log properties and changed the enlarged the log size and selected the overwrite log function. When I return to my server a few hours later my security log is full again and my log size has been reset to the orginal size before I changed it. I have also checked the active directory policies to make sure that they were not the problem as well. Does anyone have a fix for this? Help!!!! Me !!