The model is wrong! If we assume the same stress level in muscle cross sections then the strength/weight ratio of geometrically similar organisms is inversely proportional to their length/height. Thus if we assume that a man of height 1800mm has a strength/weight ratio of 1 i.e. he can lift his own weight then a man of height the size of an ant, 3mm say, should have a strength/weight ratio of 1800/3=600. Thus the ant with a strength/weight ratio of 50 is relatively weak!!!!