I know why it suddenly took up so much space on your HD and the problem is right in front of you!
http://www.uploadit.org/files/191003-untitled.JPG
Look at the center in the bottom of your screenshot where it says Cluster Size! You chose to format your drive with an insane cluster size of 4 MB! The higher the cluster size of the file system is, the higher is the amount of slack space on your HD!
This means if you have a file that is only 350 bytes in size, the file system will use the smallest cluster count, in your case 4 MB (4,096 KB) to save the file to the disk. The other space that is not taken up by the file is called slack space. So, if you save 4 small files (let's just say around 100 - 200 KB) it will actually take up 4 x 4 MB = 16 MB on your disk.
So, the only solution to fix this problem is to format the drive with a smaller cluster size. NTFS offers you the choice to format your clustters in multiple sizes, ranging from 4 KB to 4,096 KB. Best thing to do is format in the smallest possible cluster size or simply choose Default cluster size next time!