Reference: http://www.symantec.com/business/support/index?page=content&id=TECH205767
Symantec will post update to the AV Engine in Multiple Daily Virus Definitions on Tuesday May 21st 2013.
Beginning with the Virus Definitions that include the update AV Engine version 20131.1, additional files will be added. That update is expected to post in MDD1 on Wednesday May 21st.
Symantec Endpoint Protection 12.1+ and Norton AntiVirus/Norton Internet Security 2011+ customers will see 4 new index files. Symantec Endpoint Protection 11+ and Norton AntiVirus/Norton Internet Security 2007+ products will see 8 additional index files.
As a result of the additional files, the size of each dated definition folder will increase. Note the size increase noted below will be the size that is added to the dated definition folders as they exist on the disk. When definitions are distributed the files are compressed.