Shrinking an iso to as small as possible, what are the methods?

[vsub_]

You can use UMD Gen to save it as cso(compressed iso) and depending on the game(not every game is ok with this),you may remove the INSDIR files.
Dragon Ball Z Tenkaichi Tag Team
and
Dragon Ball Z Shin Budokai Another Road
are working fine but for example
Naruto Shippuden Ultimate Ninja Impact
worn work after that.

UMD Gen can also scan the image for dummy files(which just take space to make the reading of the disk more easy but most of the time are useless if you play from image)