The plane that dropped this bomb would be a larger weapon given a plane can be used as a weapon. Along the same lines aircraft carriers would be the largest weapons I could think of. Though many countries have dabbled into weather modification, then there are gas and other chemical weapons.
So is it largest based upon volume or size or weight or it being 100% dedicated weapon?
Another historical example beyond those large train driven guns the germans made would be during World War One in which it was common to dig a large tunnel under the enemy fortifications and place tons and tons of explosives under the enemy in which you would eventualy set of blowing up the enemy -
http://en.wikipedia.org/wiki/Battle_of_Messines_(1917) being a very interesting read in itself involving hundreds of tonnes of explosives. Though like landmines not all got set off and case here
http://rense.com/general47/50000lbWW1bomb.htm about a 50,000 lbs bomb (22.6796 metric ton), which is still lighter than this Tsar Bomba.
Though more background upon such explosive type bombs can be gleamed here
http://en.wikipedia.org/wiki/Tunnelling_companies_of_the_Roy... and mention is made of a of a far heavier:
""As preparation for the attack, six mines were detonated on 27 March 1916. The largest of these was at St Eloi, dug by the 1st Canadian Tunnelling Company, consisting of 95,600 pounds (43,400 kg) of explosive. This allowed the capture of St Eloi by the British 41st Division.[20]
This choice reduced the total explosive to 450 tonnes. The evening before the attack, General Plumer remarked to his staff, "Gentlemen, we may not make history tomorrow, but we shall certainly change the geography." ""
So personaly I'd not say it was the largest or is the largest weapon ever made. But certainly the most scariest.