The gluteal muscles are largest. The gluteus minimus and maximus comprise this group. They are your buttocks. A very close second is the bilateral latissimus dorsi muscles which are your "lats". They run more than half way up your back and wrap around laterally (the sides). They connect distally in the hip or coxal area. The gluteals are indeed much thicker and have more mass. Another large muscle group is the adductor group on the anterior thigh and also the quadriceps group which is also on your anterior thigh(front thigh). As you can imagine, the muscles of the legs are very strong inpart because they help to support your weight. The masseter muscles of the bilateral jaws are also very strong, and the strongest of all.