Varying Size of Full Disk Images

With my ATI 2009 I have now created a half-dozen full disk images. My question is why do they vary so much in size? My first two were about 90 million KB but the more recent couple have been only about 10 million KB. Does the different sizes have to do with whether or not I have recently defragged my HD?

My 160GB HD shows about 15GB of used space.

Thanks,
Bart