Hacking weird issue with size of games

[rise_against]

i made a backup of pinguins of magadascar (iso size is 4.5gb). now, when i transfer the game to my hdd using wii backup manager i end up with a wbfs of 700 ish mb. the game works fine on the wii but i wonder why the wbfs file is only 700mb when the iso is 4.5gb
thx in advance

 

[Cyan]

all the Wii game Discs are filled with random garbage to always be a full 4.7Gb size. (or 9Gb for dual DVD).

The game can be few MB (Mario is 400MB, there are even games less than 100MB) to full 4.7Gb ISO.
when using wbfs format, only the game data is copied, all the unused space is skipped.