Kim is using nukes as means of propaganda and showing off NK is capable of bullying his neighbours while NK population is starving. It's just a psyctoric dictator strategy to to stay in power.

According to the BBC[1], while North Korea has successfully detonated five times, it is exponentially more difficult to get the package small enough to fit on a missile.

[1] - http://www.bbc.com/news/world-asia-pacific-11813699

Why do you say that? Missing with a nuke seems like a major failure and unlikely to have the intended effect.

Blast radius + fallout radius. Either you want it as a terror weapon, and pinpoint accuracy isn't needed. Or you want it as a mutually assured destruction deterrent, and you only need so much accuracy to be credible.

Fallout radius for a 15 megaton blast, superimposed on the north eastern US. Anything with numbers (roentgens) above ~500 is fatal, though less than that isn't harmless either: http://nikealaska.org/nuke/CasBravM.gif Much, much bigger than what the DPRK has tested, but the principal is the same. Low tech SCUD level accuracy is fine.

The Nagasaki bomb missed by 3km. It was similar in size to what the DPRK has tested.

And quantity makes up for quality.