Well, McNamara is wrong, I think he remembers incorrectly, or it was a very,very secret project:
"By contrast, the largest weapon ever produced by the United States, the now-decommissioned B41, had a predicted maximum yield of 25 megatons, and the largest nuclear device ever tested by the US (Castle Bravo) yielded 15 megatons (due to a runaway reaction; the design yield was approximately 5 Mt). For comparison, the theorised supervolcanic explosion of Lake Toba, in Indonesia, some 75,000 years ago, is estimated to have been equivalent to 1 gigaton of TNT, and therefore twenty times larger than the detonation of the Tsar Bomba device, and the asteroid impact which formed the Chicxulub Crater, was an event larger than Tsar Bomba's yield by some six orders of magnitude; it released an estimated 500 zettajoules (5.0×1023 joules) of energy, approximately 100 teratons of TNT, on impact."
Anywhere I read, the Tsar Bomb was the biggest ever exploded. McNamara might be saying the US designed a bigger one, but I don't think the US tested it....