Sharepoint 2010 farm full backup

I do full backup to sharepoint 2010 farm, in logs write

Estimated disk space required: 94,251 MB.
Backup
Backup Method: Full
Top Component:
Configuration only: False
Progress updated: 5
Backup threads created: 10

at the start ofthe backupisthe approximate size of94,251 MB (about92.05 GB),and in the endallbackupfilesweigh 19GB.Atthe start andat the end ofallsizesbackupfiles do not match.Why?