gtar backup taking longer time

Dear All,

we are taking gtar backup by using "gtar -cvf - bnk| pigz -5 -c > /test.tar.gz" command. we take complete backup i.e. bnk full. The problem is that if we restore the backup ( test.tar.gz file) and take backup, then it would take about half an hour. then, backup time will increase each day abnormally like after 15 days, it would take more then 2 hours while data size was increased only from 170 to 175 GB only. But, if delete bnk and then restore the data ( i.e. of size 175 GB) and take backup then again it reduces backup time to half an hour. What may be the reason?

Best,
pokharel

Any suggestion??