Differential Backup question

I've just updated TI 8 with the new version 9 and i've tried to manage with
the new feature about differential backup. I cant understand the advantages of this option because i've created a full backup and then a differential backup from it... but the size of the new differential backup file is the same of
the full backup file. Differential backup shouldn't backup only the changes from full backup ? I've done something wrong ?

 

Same here, tried it several times and the differential is always (slightly) bigger than the full. Incrementals work fine.