Hi Erhan,
We cannot process the image file at all until it is uploaded so, the file will be whatever size it is. So "Full Size" just means whatever size the file is when the user uploads it, it could be a big file or a small file, we only can find out the size after it is uploaded.
But no matter what size it is, the original uploaded file is considered "Full Size".
After it is uploaded we can resize it.
What I'm saying is we could make a config option or a feature setting to automatically delete the original file after we make the thumbnail and web size. This is what I put in our project tracker and this can help if disk space is a concern.
Best,
Joe