VM Backup Size/Durarion

Hi,

I setup VM backup recently and this is how it went:

1st day - 1 hour 15 min - backed up 61000mb

2nd day - 5 hours 25 min - backed up 350mb

3rd day - 2 hours - backed up 500mb

4th day - 2 hours 45 min - backed up 3900mb

My question, is it normal for backup to vary in duration so much? and is it normal that 2nd/3rd/4th day it did not backup full 61gb?

Thanks.