2 things. They could be making smaller versions for loading on missiles (my guess) or preparing primaries for thermonuclear weapons which need relatively small 1kt yields.
I agree with your statement. The DPRK could hand out a lot of havoc in the region with short range solutions. Or they could be working towards something very BIG. Bottom line, who are they trying to either force into a castle position or do they actually think they can maneuver into a mate position? The norks are hard to anticipate when hungry (compound too reports of the new kid not being received well, where does THAT leave “us”?).