http://download.yousendit.com/687BFED6742769DD
^^ 1.2 gig worth of ntai log files compressed into a 44 meg archive.
For a real life analogy...
Imagine a 2 liter bottle is empty. You stick it in the trash, it takes up a lot of space. Now imagine you take that same 2 liter bottle and squeeze the air out of it. That's kinda like compressing a text file.
Now, Imagine the bottle has about 3/4 liquid in it. Once again, you squeeze out the air, but the bottle still takes up more space in the trashcan than the empty one. That is kinda like compressing a bmp file.
Meh, useless info to a point, but it's surprising how many people don't already know this.