This is likely down to the number of definitions you're retaining on your SEPM. Please see the below article on disk space management options on the SEPM:
http://www.symantec.com/docs/TECH96214
#EDIT#
Best Practice articles on the retained defs can be found below too:
http://www.symantec.com/docs/TECH92225
http://www.symantec.com/docs/TECH92051