At a very high level bombs are trying to convert as much of their fuel into energy as possible. The ideal bomb consumes most of the fuel and produces a lot of very nasty extremely short lived nuclear waste thus making a big detonation over a small fraction of a second. Waste products should be short lived isotopes to make them even more deadly weapons.

Meltdowns aren’t controlled reactions the waste includes perfectly useful fuel, short and long lived waste products, plus a mix of things such as control rods and the walls of the reactor etc. https://en.wikipedia.org/wiki/Corium_(nuclear_reactor)

There’s a few other effects such as mushroom clouds moving material away from the blast location, and reactors containing more nuclear fuel.