Images transfer over USB does not clear up free space?

Hello,

After my iphone has been filling up recently, I decided to move over most of my photos to my (windows) computer.

I simply did this how I usually would, connect the usb cable to the computer, go into the DCIM folder and Cut+Paste any (or all) photos from my phone to a folder on my desktop.

After the process is done, all the photos are also actually removed from my gallery app, HOWEVER the free space does not seem to clear up almost at all.

I have copied over 6 Gb of images already (that are no longer available on my iphone as well) yet I only have about 400 mb of extra free space?

And no, my trash folder in the gallery app is empty I already checked that.

There has to be some cache of this somewhere? I doubt there is so much compression on the iphone going on that 6 Gb turns into 400 Mb