why is Imaging data folder size not equal to the total size of contents? (Using FTK imager Lite 3.1.1)

I have a folder with excel,word,and nsf e-mail files inside and then I got imaging by FTK Imager Lite and created AD1 files.
When I select the Original folder properties and check the folder data size (not size on disk), it is 950GB .
However, After aquired image ,if I select AD1 image data folder and check the total sizes of the files (not size on disk), it is 690GB .

This should be caused something different. My question is why is the size of a AD1 image file folder not equal to the total size of original contents?

Please advice. Thank you in advance.  

- mscotgrove

Did you enable/select compression?

Thank you for your advice mscotgrove.

I didn't know "compression flag" . Is the compression flag default value True? (compress enable?)

This is my first acquisition server data and using FTK Imager Lite.

Thanks.  

- mscotgrove

With much of forensic investigation it is understanding tools, what they can do etc etc.

Look at the program again and work out this process can be comnpressed.

I do not use FTK but I am aware that many image formats do have comnpressed versions.

mscotgrove
Thank you very much your advice.  

default is an initial compression level of 1 is on and listed as "fastest" go to 0 for no compression.
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- armresl

default is an initial compression level of 1 is on and listed as "fastest" go to 0 for no compression.

armresl Thank you for your advice.

I checked default value of "Contents of folder" compression:level 6
Additionally,physical drive acquisition compression :level=0 (default)

Thank you.