Why can thermonuclear weapons release far more energy than fission bombs?

Thermonuclear weapons can release significantly more energy than fission bombs because they utilize the process of nuclear fusion, whereas fission bombs rely solely on nuclear fission.12

In a fission bomb, energy is released when a heavy atomic nucleus splits into lighter nuclei. In contrast, a thermonuclear bomb uses a two-stage design: a fission bomb acts as a primary stage to generate the extreme temperatures and pressures required to ignite a secondary stage containing fusion fuel, such as deuterium and tritium. During fusion, light nuclei combine to form heavier helium nuclei, losing a small portion of their mass in the process. This lost mass is converted directly into a massive amount of energy according to Albert Einstein’s formula, $E=m{c}^2$.1234

Furthermore, in many modern thermonuclear designs, the high-energy neutrons released by the fusion reaction induce additional fission in the surrounding uranium container, which often accounts for a large portion of the total explosive yield. This multi-stage process allows thermonuclear weapons to reach explosive powers measured in megatons, far exceeding the kiloton-range yields typical of pure fission weapons.1