Things like 4Gb to 18MB, What makes this kind of compression methods possible?

[CompressIt]

A super-noob in encoding, right here. I downloaded a Wii ROM of Super Mario All Stars - It was another game though - that was just 18MB and when I extracted the .rar file, I found a functioning .iso of 4.5GB.

I'm really invested into knowing what makes this kind of compression methods possible? It's fascinating. Now I a little about how their file type is much more important issue, But can anyone give me a simplified explanation about this?

Here is a picture:

[JamesB]

It's a disk image (a DVD in this case). Your hard disk has the same number of bytes regardless of how many you've used - the others are all just initialised to some empty value - eg 0. The same applies here. Store 18Mb on a 4.5Gb DVD and 'lo, you can trivially compress the 4.5Gb DVD image to 18Mb. I expect it's actually larger than 18Mb - say 50 Mb used, compressed to 18, but the point is the same.

[CompressIt]

Thanks for your response. Now that I took another look I see, The game is actually 90MB.

[Jaff]

Thanks for your response. Now that I took another look I see, The game is actually 90MB.

With lots of uncompressed data...