Strange Disk Space Issue

Hello.

I have a 13" Macbook Pro, running Catalina and a 500Gb HD.

I currently have 300Gb free disk space left. I recently duplicated a 50Gb folder, meaning the free disk space should have reduced to approx 250Gb, but it didn't, so to further test this, I duplicated it again a number of times (renaming all contents each time).

By the 6th time of duplication and renaming, my HD should have told me 'disk full' but it didn't. In fact I went on to duplicate it 10 times (500Gb) without any error at all. This should have exceeded the 300Gb free space I had.

Can anyone tell me why?