Nuclear fuel is very energy dense, by volume or by mass, compared to anything else we've used in the past. The energy density of a nuclear fuel for fission is about one million times greater than fossil fuels using today's fission technologies and perhaps 10x further using next generation techniques. Fusion fuels are about 4-10x higher in energy density than fission fuels. But this is in terms of energy yield per mass of fuel. The energy delivered to the end use per mass of fuel will likely be almost equivalent once a fusion reactor goes online. Fundamentally, nuclear fuel's higher energy density should make it much easier to build machines to harvest and use it.