The thing is, there isn't really a scientific delineation. Atomic is a historical term that generally referred to fission-only devices, but that was mostly a consequence of fusion having not yet been achieved rather than any technical meaning. Hydrogen bombs generate energy from fusion of hydrogen, hence the name. Pretty much all modern weapons except for some janky 3rd world attempt will contain hydrogen, so that's not really a meaningful term anymore either. A nuclear weapon is anything that uses nuclear (rather than chemical) process to generate energy.

Fat Man was nearly the size of a Smart Car and delivered O(10 kt), while modern weapons are much smaller and with higher yield (though that yield is really more driven by military strategy and availability of materials than it is the ability to get more yield. The truly gargantuan bombs of the 60s have gone out of style because for a variety of reasons.) You could comfortably cradle a modern weapon in your arms, if it wasn't absurdly heavy due to the density of materials used.