I'm not sure if Linux filesystems work the same way, but with FAT a sector is either used or free. You can't have it "50% available". If you have your hard drive divided up into 32KB sectors (like FAT16), your 5 byte text file will use up 32KB of disk space. If you have FAT32 with 4KB sectors, that same tiny text file will take up only 4KB of disk space, giving you 28KB more free disk space. Defragging doesn't actually give you any more free space...
Assuming Linux works the same way, a bunch of small files in an i-bay with the same total size as a single big file will actually cause more disk space to be used...