In general, engineers will size the fuse to be the maximum load x wiggle factor. However, many will size the fuse based on the lowest current capability of the components. This means that if an amp draws 5 amps peak during operating, but the smallest component won't burn at 10 amps, the fuse will be 10 amps.

Efficiencies of amps varys widely, too. I've had a few amps that made better heaters than amps. Once I had a Delco amp that, insanely, drew maximum amps at 0 volume, and dipped inversely to volume! (I think this was done to compensate for the crappy Delco speakers that would buzz if hooked to a normal amp. The stereo would retract the speaker cones to maximum and operate them inversely to normal systems. It did sould nice at low volumes.)