The B-36 might have been a heavy aircraft when it first flew, but it's not even close to the heaviest aircraft, even a B-52 weighs more, a C5A can lift more weight than the B-36's empty weight. The Antonov 225 payload is more than 3 times the B-36's max overload weight.
But if you're talking about just size alone, the Ant-225 is the only one larger.
The B-36 first flew in 1946, that's 6 years before the first H-bomb. It design was started in 1941, to be able to bomb Europe from the USA, in case Britain fell and we'd have no closer airbases.