EDIT: Considering the power outage, try doing a chkdsk /f and see if it find errors, else you'll have to copy the files all over again.
-Original post by me:
Two possibilities: Compression using the default windows .zip interface and sector size. I would go with the latter. It's like this:
Compression is fairly easy to explain. "Size" dictates what the actual size is, and the "size on disk" dictates how much space it is occupying the hard disk after being squished in to a very small size.
Sectors are a bit more complicated. Your hard drive is divided in to many, many things. The smallest and most important are the sectors. Think of sectors as honeycombs in a bee's hive. They combine together to form an actual hive of honey.
Now, Windows 7 has a default sector size of 4KB. That means that for every block of data, it divides it into a 4KB pieces. Now if you have a 6KB file, the first 4KB will occupy sector 1. The next 2KB will occupy sector 2. You cannot add another file on sector 2 since it is already occupied by a file. Considering that, file 1 is actually occupying 8KB even though it is only 6KB.
To give you an example, I'll show you a screenshot of my PC. My sector size is at 32KB (the bigger, the better in performance :P)
See how even if it's 1 byte it takes up 32KB? It's because of my sector size.
That's a loss of 32,767 bytes. Of course, the effect stacks up. Now see what happens when I copy that file 237 times.
Hopefully that should give you a basic understaning on why there's a big difference on "Size" and "Size on disk".