Why is the actual size of a file different from size on disk?

Properties of any file/folder usually gives a larger value of size of disk than the actual file size. Why does this disparity exist?
The reason there is a discrepancy between the size a file appears to be and the size it takes up on the disk is because of the disk's filing system. [1][2][3][4][5] The space on a disk is divided into "allocation units" or "clusters," which are the smallest unit the filing system can address. [2][3][4] Each file is saved in one of these clusters, which means that although the disk may appear to be half-empty, every cluster is probably being used. [1][3][4] Because each cluster is being used, the total size of the cluster is the size on disk, which will always be larger than the actual size of the file. [3][4]
Actual file sizes differ from the size they appear to be on the disk because of the disks allocation unit size [2]. These allocation units can not be subdivided into smaller blocks for smaller files [3]. The disk will give the smallest file the smallest space it can but this means that sometimes it will take up more space than it needs [1,4,5]. This is because it needs to keep track of all the files it has inside. If it did so precisely at the amount of kb's the small file used a lot of the space used in the disk would be tracking down the files [2].