@amelia
Let's first explain a common misconception about Windows and how it handles disk allocation: The size property is logical, it shows the total amount of bytes that the files will take once they are fully downloaded. Size on disk is the actual size they are taking right now. This is the normal behavior, it works like this regardless of whether you're using an HDD or a SSD.
If you are seeing 28GB and 12GB respectively, this means that you have partially downloaded files sitting in your disk. Those files alone are taking 12GB, but when they are fully downloaded, they will take up 28GB.
So no, you have not freed up 16GB, those 16GB are what you are still left to download in order to complete those files you have.
If you had more files in that folder and they were successfully deleted, this means that the folder was once taking more than 12GB on your disk.
To avoid similar problems in the future:
Always stop the torrent before moving or deleting its files:
Regardless of whether a file was fully downloaded or only partially, trying to delete it while the torrent is running might prevent them from being deleted. Incomplete files are locked by the client to prevent them from being deleted or moved before completion, while completed files can be locked if they are being seeded in the moment.
The maximum amount of bytes you will recover when deleting a torrent can only be the value reported by Size on Disk:
Some torrent clients will pre-allocate all the torrent size, so you will see Size on disk: 28GB even before you have downloaded a single byte from that torrent. This is optional, most clients allocate bytes as they are downloaded.
Without pre-allocation, if you have a 28GB torrent and you only downloaded 5% of it, the size on disk will be 1.4GB. Completely deleting this torrent therefore will only free up 1.4GB.
With pre-allocation enabled, on the other hand, a 28GB torrent will immediately take 28GB of effective size on disk, even without completing any file. In this case, completely deleting the torrent will free all 28GB that it pre-allocated.
And finally, if you stop the torrent in your client's UI, you can go back and delete that folder at last.