flourocarbon leaders

I am confused about flourocarbon leaders. If they are invisible to fish why do they come in different pound tests? I mean the invisible man is still invisible, wether he is fat or skinny, so why wouldn't I want to use a heavy flourocarbon leader?

But they are still differing diameters. Think about it...would you have the same action out of spinner tied to 8 pound as the spinner tied to 50 pound?? Nope.

yeah, thats pretty obvious...but why drift 8 lb when 12 or 15 would be about the same action and yet still invisible.