Size is the actual size of the file. Size on disk is the actual amount of space taking up on the disk.
The disk consists of tracks, sectors, and cluster. (see quoted below)
Let your drive size > 2GB, so your cluster size is 4 KB and if your file actual size is 220,810 B,
the size on disk will be 216 KB because it is occupying 54 sectors.
size on disk >= size
4 * 1024 * n >= 220,810
4 * n >= 215.6
n >= 53.9
n = 54
size on disk = 4 * 54 = 216 KB
Here is the default size usually for Windows.
Drive size (logical) | Cluster size | No. of Sectors
-----------------------------------------------------------
<= 512 MB | 512 bytes | 1
> 512 MB - 1 GB | 1 KB | 2
> 1 GB - 2 GB | 2 KB | 4
> 2 GB | 4 KB | 8
引用:
http://computer.howstuffworks.com/hard-disk7.htm
A typical track is shown in yellow; a typical sector is shown in blue. A sector contains a fixed number of bytes -- for example, 256 or 512. Either at the drive or the operating system level, sectors are often grouped together into clusters.