Why size on disk is smaller than Size?

Answer 1

The size is exactly what it sounds like - the size of the file. The size on disk however is not quite as simple. Before I can answer that, we need to review some file system terminology. I'll be using the NTFS file system in these examples, but the same principles apply for most (if not all) other file systems. A cluster is the smallest amount of space that can be allocated on the file system. Cluster size indicates how large the cluster is. The default cluster size for an NTFS volume for a majority of drives around today is 4KB. The size of the volume does change the default cluster size, but for now, we'll assume 4KB. Let's say you have a file that is 1MB. Now, 1MB = 1024KB. So if we divide 1024 by 4 we will get the total number of clusters this file will occupy on the hard drive. In this case, that number is 256. Since these figures divide evenly with no remainder, the size on disk for this file is also going to be 1MB. Now let's use 1.2MB as our example file size. 1.2MB = 1228.8KB. Again, we'll divide 1228.8 by 4 to come up with 307.2. This means that the file will take up 307 whole clusters, and 20% of the last cluster. Since you can not allocate space smaller than a cluster, the entire cluster is considered to be in use. This is where the size on disk value is important. To calculate the size on disk, we have to round the number of clusters up, and then multiply by the cluster size. We'll take 308 * 4 to get 1232KB. In this case, the size will be 1228.8KB, but the size on disk will be 1232KB. So to summarize, the size value is the actual size, but the size on disk measures the amount of space each cluster of the file is going to occupy.

Answer 2

The other way around is more common, but this could happen with certain operating systems and/or file systems when storing a "sparse" file.