MacMini Firewire Cable Confusion - help!

Could someone kindly point me to the proper firewire cable type I need to connect my G5 to my MacMini? I have a firewire cable on my G5 which runs to an external HD, but that has a completely different end on the cable for the peripheral. And the end that does fit my G5 firewire port doesn't fit my mini, it's too big... My G5 is supposed to support firewire 800 just like the mini, so why is the mini firewire port too small for the cable I have? I assumed all 800 firewire cables would be the same size. Im quite confused and have no idea what I'm doing. HELP! (My MacMini is a 2010 model, doesn't have Thunderbolt port). Could my G5 really be a firewire 400, even though the System Profiler says under Firewire supports 800, hence the diff cable connection size?