In a prior version of the TTT Gallery, on which the sub-project Gallery is based, there was a configuration option of "Keep Source".  It basically determined whether the "source" version of the image was kept. Basically, after upload, a thumbnail and a resized gallery image are created from the original.

The "Keep Source" option allowed to delete the original image (which could be several MB with todays cameras). It now has a hard coded value of "True" which means much disk space is taken, particularly if "Download" is disabled.

Look for references to IsKeepSource or mKeepSource in the source code and you will see it. I'd like to brong that option back so I can save on my disk space.