@amelia your brain is tricking you, you think you downloaded 28TB and after deleting the files only 16GB were deleted. What happened in reality is that, you only downloaded 12GB all this time, the 16GB only refer to the amount of bytes that you never downloaded.
If you have deleted the folder with all its files, you have freed up all the space that those files were taking.
Like I explained before, in your head you are thinking: the folder has 28GB, if it's taking 12GB on my disk then I must have deleted 16GB.
That is not what is happening, the files that should be taking 28GB from your storage are only taking 12GB, because you never finished downloading them, you only downloaded 12GB out of 28GB. You still have 16GB left to download. So if you delete the folder right now, it will free 12GB. If the files are taking 12GB on disk, then 12GB is all there is to be freed once you delete them.
To put it more didactically, let's pretend you have a disk with 100GB of free space. You download a torrent that totals 30GB. The torrent folder size will say 30GB, and size on disk close to 0 bytes. You still have 100GB of free storage in this moment. As the download progresses, the free space will decrease. Once you download 10% of that torrent, folder size will still show 30GB, but size on disk will have increased to 3GB. Your free space will now be 97GB.
If you delete all the torrent files at this stage, you will be deleting those 3GB, going back to 100GB of free space.
So to be as clear as day, if you have successfully deleted the folder and its files, you have already recovered all the space that there is to be recovered. There is no need to do anything at all, no de-fragmentation, no nothing. If you were expecting to free more space than what you actually got, you likely had a misconception of how incomplete torrents are stored.
Reinforcing: if you can still see a folder that says "size on disk: 12GB" you have not deleted all files! Once you delete that folder, you will get 12GB back.
If you still think there's something that I failed to explain, feel free to ask and I'll do my best.