Faultyness

Hello. I have a problem with burning DC games to a CD-R disk. Here are the details:

After watching a tutorial, I went to download the game file.
I downloaded Sonic Adventure 2.
I also downloaded DiskJuggler.
I have a 700 MB CD-R, and my goal is to burn SA2 onto the disk so I can play it on my DC. Now here's the problem. At the website (where I downloaded it from), it clearly states that the file size was 642.51MB (from what I remember), and I download it. 4 hours later, I take a look at the file.
The file says 766 MB. WHAT??!!
I clearly downloaded it, and it tells me something different?!
Can someone explain what I am doing wrong?

Oh, and it's not only this file, it's a lot of others, too. Please, can someone tell me what I am suppose to do? I would appreciate it.