Significant difference in size between raw data and acronisised data

Harry Putnam wrote:

ATI 2013 on win7 x64
I just noticed a phenomena I hadn't noticed to this extent in the past.

Created a partition backup. The size on disk before backup is 104 gb. However once the backup has completed, the backup itself weighs in at 129 gb.
A 25GB difference.

I did use maximum compression, but I usually do. I have that as the defacto setting.

Compression is supposed to make the backup smaller.
This seems like a pretty hefty difference. Should I be concerned?

Please ignore this post.... I blindly overlooked that I had somehow included a second
partition of 129 GB.

So combining the two:
104
129
====
233
Ok, that's looking a mite better.

What really happened was 233 gb was squeezed down to 129 gb... wow.