What is the largest muscle in the human body?

The gluteus maximus, and it is large because of how we walk. It is not primarily for propulsion; its main job in humans is stopping the trunk pitching forward when the leg takes the weight, which is a problem quadrupeds do not have. Chimpanzees have a much smaller one relative to body size, and the enlargement shows up in the fossil record alongside other running adaptations.

Largest depends on the measure, as usual. The gluteus maximus wins on volume; the quadriceps group is the strongest; the soleus in the calf produces the greatest sustained force, holding you upright all day; and the masseter in the jaw generates the most force for its size. The heart is the most durable and is not skeletal muscle at all.

Sitting is what undoes it. Prolonged sitting keeps the glutes lengthened and inactive while the hip flexors shorten, and the muscle becomes poor at firing when needed - which is a real clinical pattern with an unserious name, gluteal amnesia, and which shows up as lower back and knee pain because other muscles take on work they are not shaped for.