Incremental BU-WARNING about defrag

I have been puzzled about the size of my Incremental backups which appear to be about the size of the origional full backup. I have now learned that if you carry out a defrag prior to an incremental backup the size will be very large.
I have decided to carry out a defrag only before a full backup and have no defrag on the incremental backups.
I have also learned that the maximum size of file for a DVD disk should be kept to a max of 1.9GB. I was expecting to store 4.7GB!
Thie information may be helpful to others
Blair