While nuclear weapons and nuclear reactors are superficially similar, they're generally vastly different when it comes to most things.

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Little Boy and Fat Man (and generally any nuclear weapon) involved a fairly efficient fission reaction in a small, highly enriched block of fissile material. Little Boy contained only 140 lbs of Uranium, while Fat Man had an even smaller payload - just 14 lbs of plutonium. To get the kind of rapid, explosive power that gives us a nuclear bomb, the reactions are very efficient compared to a reactor that's just providing power. In both weapons, about 2 lbs of the core fissioned to produce the force of the bomb. Compare that to Chernoboyl's nuclear reactor. Nuclear reactors work with fuel that's less enriched than a bomb, but also have significantly more fuel in the reaction. At Chernobyl, Reactor 4 alone had 180 tons of nuclear fuel. A much smaller fraction of the fuel is fissioned away at any given moment, producing a (comparatively) less energetic reaction that's feasible to contain. So Chernobyl had significantly more radioactive material on hand during the disaster, while Little Boy and Fat Man not only had significantly less from the start, but fissioned away much of their radioactive material.

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Then we've got the mechanisms for fallout to look at. Fallout from nuclear weapons is highly variable and is dependent on things like the location of the detonation, specific construction of the bomb, and yield of the weapon. Ground bursts tend to have the highest amount of fallout, as the detonation can most easily interact with debris on the ground. Air bursts like the ones in Hiroshima and Nagasaki, however, tend to "pick up" and irradiate significantly less debris. The Chernobyl disaster, on the other hand, benefited from some of what can make ground bursts so irradiating in that it had a lot of surrounding non-fissile material to irradiate and throw into the atmosphere. The reactor meltdown and subsequent burst of the reactor saw 5% of the reactor mass - 7 tons of fissile material - released into the atmosphere, and subsequent fires would create particles that would get irradiated and thrown up into the atmosphere by the heat of the fire and meltdown.

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So ultimately what happened at Chernobyl was that it released significantly more radioactive material into the atmosphere both in the form of reactor mass and irradiated particles from the powerplant. Meanwhile, Little Boy and Fat Man had comparatively very little fissile material left over to distribute as radioactive fallout, and were detonated in such a way that it happened to minimize fallout.