Actually, I think the OP isn't explaining himself/herself clearly.
The actual files are not being reduced in size at all. However, they could be split into multiple pieces and stuck in different parts of the hard drive.
For example, if you just ordered a pizza, sliced it up, and put each slice on a different plate. You still have one pizza, but the slices are in different places instead of in the box itself.
Let's say you ordered six pizzas for a party and put each slice on a different plate. You go out for a beer run, but you didn't tell anyone how many pizzas were ordered.
One of your buddies wants to know how many pizzas were ordered. They would have to count each individual slice, know how many slices come with one pizza, and do the math from there. A pain in the ***, isn't it?
File fragmentation is the same thing. Pieces of your files are scattered around the platters of your hard drive. Your computer only shows you the file. If you try to access the file, the computer will load the file (and all of its pieces) into RAM. To the end user, it's still just one file of the same size. Behind closed doors, the file's pieces are not in the same place, so it's more of a pain in the *** for your computer to find the pieces (hence, taking longer and stressing the hard drive).
In this case, you're asking the wrong question. It's like the person paying for the pizza asking how many slices of pizza there are. He/she doesn't need to know. He/she just needs to know how many pizzas were ordered so they can pay for it.
In your case, you don't need to know how small the pieces of the file are. All you need to know is that your file is there and that its contents will be there when you access it. You defrag to make life easier for your computer.
