A given sized wire can only carry so much current before heat develops. The purpose of the fuse is to be the weakest link, so if too much current is drawn through the circuit, the fuse will fail first, thus protecting the circuit from overheating. A light bulb is basically a very thin conductor carrying a lot of voltage such that it is caused to glow, producing light, getting hot as a result.
Having a 30 amp in a 7.5 holder will keep the circuit from blowing, but allow 4 times as much current to go through the circuit, allowing overheating. That's not safe. That's why different sized cartridge fuses are now made different lengths so one size won't work in another sized holder.
Having a 7.5 amp fuse in a 30 amp holder will be safe, but inconvenient, because it will probably keep blowing because it will only allow 1/4th the current that the circuit was designed for, so devices stop working because the fuse keeps blowing.
Some day put all the right sized fuses in all locations. If a fuse keeps blowing, correct the problem rather than increasing the fuse size.
Be safe!