Because for mass to be critical it has to be big. A tiny bomb means extremely high tech. That idea is fake news. Meanwhile, the tests registered up to 5.2 on the richter scale a few hundred miles away, which would have to be big. The "small" claim is psychological warfare on us. We didnt have the technology to build a bomb a fraction of the size of Hiroshima until many, many, many years later. The small bombs are considerd the most advanced. Like comparing the giant refrigerator computers to an iphone.
http://www.businessinsider.com/seismologists-eavesdrop-on-north-koreas-nuclear-tests-2017-7
