Bigger wire has less resistance per length. A fuse is basically a wire designed to clear when overloaded.
Fuses and breakers are really to protect the wire from overloads.
A 12ga wire is good for a max load of about 20A. We won't get into free air load vs. in a bundle or conduit (it is rated at less).
Sooo.....a 12ga wire can carry about 20A load. So the fuse would be equal or less so you don't overload the wire and start a fire.
A smaller fuse on a big wire can then help protect the equipment.
Small fuse (amp/A rating) is the same as a lower load. The good thing is it clears when overloaded but no fire.
Downside is voltage drop since it is not designed for a big load.
So a big wire, then fuses that protect the equipment within reason. This gives max power (less voltage drop) to the fuse.

Hope I didn't muddle things.....