Easy-Peasy. When two atoms fuse, the resultant weight is less than the sum of the atoms' weight before fusing. The resultant energy comes from the difference in mass, and is given by the well known equation E= m * C^2, where m is the change in mass, and C is the speed of light. The first atomic bomb dropped on Hiroshima was a fission bomb. The second bomb was a fusion bomb. That was actually a two stage bomb. The first stage was a fission bomb. It supplied enough pressure to force hydrogen atoms together to fuse them. The resultant energy created the second stage of the explosion. It's possible to create so much energy from a hydrogen bomb that it actually blows the atmosphere into space. At that point, there's no reason to make them more powerful.