why increase all of my diff backup size after 3rd day

Dear all

Hi

i have  created many  job in backupexec that get  one full backup(vmdk) in a month and after that get diff backup every day(vmdk) in most of my backup 2 of my diff backups size are so low for example first day is 25M , second day is 30M  but after second day all af my diff backups size so increased to 1.5G and 2G and 2.2G , .... i am sure after second day no changes doing on my source servers .

All of my backups Diff size will increase after second day i have attached backup set from 3 server

why ?  Is this Backupexec architecture?

How can solve this problem?

Warm regards

Babak