NO NO NO WHY THE HECK ARE THERE SO MANY PEOPLE WITH THE STUPID MISCONCEPTION THAT DEFRAGGING GIVES YOU MORE SPACE?

Defragging rearranges stuff on the disk. On a FAT/FAT32 formatted HD which most people will Windows probably have, rearranging is rearranging, nothing more. The same number of file blocks or clusters used to store your stuff will be used at the start of defragging, and the same number will be used at the end, just in a different order for extra speed in accessing and creating files. You gain nothing in terms of space with defragging such disks. I don't think I can explain it any simpler than that.