How can there possibly not be enough room for a backup?

I've got the latest version of TWRP and trying to make a backup. It says there's not enough room. I have the SD card selected and it has 7 free GB of space. 13.3GB of internal storage is used. Is there no compression? This was working fine until I upgraded to Marshmallow.

Also I'm a bit surprised to see internal storage so full, I just installed Marshmallow.

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