The fuse is there to protect internal components. If you just add a bigger fuse, you'll likely wreck your amp. It wasn't meant to put out that kind of current.
When you wire lower, there is less resistance and your amp puts out more power. Voltage doesn't change, amperage does. More current is allowed to flow. Think about how small some internal components of your amp are..
Also, if you fuse larger than your wire is rated, that defeats the purpose..