Yes, it was supposed to be a bigger bang. However, as told in Burning Bright, I believe, they launched it into the building the bugs had claimed as their new hive just before they completed a massively powerful barrier ward around the building. The ward snapped shut just before the blast, containing it and amplifying the radiation inside the Cermak crater. In other words, the tac-nuke should have vaporized the entire neighborhood, if not most of the CZ, but the ward kept things contained. Magic + Nukes = Strangeness Happens.