An important difference between a fission bomb and a thermonuclear bomb was that except in its fission trigger, the thermonuclear would require no critical mass. As a fission bomb exploded, it disassembled its critical mass, at which point fissioning stopped. This disassembly process set a natural limit to the size of the fission explosions of about one megaton. A thermonuclear explosion, however, if it could be made to ignite and sustain thermonuclear burning, would proceed like a nuclear version of a chemical explosion, continuing to burn so long as it had access to thermonuclear fuel. The stars - thermonuclear furnaces thousands and millions of times as large as the earth - made it obvious that there were no inherent physical limits to the size of thermonuclear explosions.
...
During the war, Serber remembers, "on
Edward Teller's blackboard at Los Alamos I once saw a list of weapons - ideas for weapons - with their abilities and properties displayed. For the last one on the list, the largest, the method of delivery was listed as "Backyard." Since that particular design would probably kill everyone on earth, there was no use carting it elsewhere."
http://en.wikipedia.org/wiki/Space_Pen
