I'm inclined to think that just a few ohms is more than enough to create a big heating problem. Look at heating power this way: P = I²R, voltage doesn't really matter.
These fuses are 15 amp fuses, so presume for the sake of figuring that the load current is 15 amps (I know it isn't). If the internal resistance of the fuse is, say, 5 ohms, then you have 15² x 5, or 1.125KW of heat being generated inside the fuse. More than enough to make it very hot.
Do the same figuring on a 100 amp fuse, a 400 amp fuse, etc. You get into really big numbers quickly.
That's also how loose, high resistance connections at wiring devices, CB's, etc start fires. The high resistance doesn't have to be very high, but still many times higher than a good connection or conductor. The resultant heat does the damage, not arching (at least initially).
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