This is to do with Clusters and Sectors on your hard drive. Sectors are the smallest portion of the hard drive that you can write to and clusters are a collection of sectors that are addressed in the File Allocation Table (this is an explanation for FAT, NTFS is slightly different but still should explain why you are getting the different sizes).
Anyhow, if you have a sector holding 512Bytes and you have a Cluster of 8 Sectors, you would get a 4K Cluster. This would mean that this is the smallest amount of data that can be stored on the drive.
Therefore, if you have a small text file of 600bytes (1 and a bit sectors), you would see the file size as 600bytes but the space on the disk would still occupy the full 4k because the FAT cannot address the additional space as it can only do the 4K chunks. NTFS works slightly different in that the chunks are smaller (so you get less wasted space if you have smaller files) and the way it references and keeps track of the space (uses a Master File Table instead of the FAT)