From my recollection, even in a fusion bomb, most of the energy comes from the fission. I don't fully remember/understand the physics involved. But apparently the fission reaction triggers the fusion reaction (that part I get) and then that fusion reaction in turns boosts the fission reaction.

So my takeaway is that fission bombs are not very efficient to begin with. You're after all trying to extract as much energy as possible as quickly as possible from a system that is literally self destructing. We do not have that constraint with current fission plants, the plant is designed to run in a steady state for years, the fissile materials stay in place and my intuition is that this significantly helps efficiency.