A standard ISO file usually sits around 4.3 GB , which was the standard size for a DVD-based PS2 game. A "highly compressed" version uses advanced archiving tools like 7-Zip or KGB Archiver to strip out "padding" data or compress the files into a much smaller download size, sometimes as low as 500MB to 1GB .
When searching for "highly compressed" links, be cautious of: dragon ball z budokai tenkaichi 3 highly compressed link
Use a tool like 7-Zip or WinRAR. If the file is "highly compressed," extraction may take longer than usual as the CPU decompressing the data. A standard ISO file usually sits around 4