The Hiroshima bomb exploded above the city, thus causing more damage.
A underground nuke can be contained. The mini nukes there were semi contained or subsurface. Had they been at the top of the towers, yes, it would of wiped out a lot of NYC. Being they were mostly underground, the granite absorbed most of it. Being it can't be explained all at once, when you have the time, if you can review these articles or videos:
Engineers at the WTC discussing.....
SH&E at Ground Zero
How the undgnd detonation works. 7,8,15,17,22,23,25,27,30,35,39,40,56,68
https://www.osti.gov/biblio/4612556
Pertaining to investigator Asteneh. Hot spots map.
Context of 'September 12, 2001-February 2002: Witnesses See Molten Metal in the Remains at Ground Zero'
A physicist explains how the WTC were brought down. Can match some info to osti report.
https://www.youtube.com/watch?v=Ry4UWQjJnSc
Temps at this test.
https://mashable.com/2016/11/06/mississippi-nuclear-test/#.xSB4DmzYiqK
Water wouldn't put out the WTC fires. This foam had to be used. Still took months.
http://www.pyrocooltech.com/