This paper presents our efforts to explain why mammals have large thigh muscles while insects have small ones. After a discussion of this observation a definition of body foot ratio is defined which describes how animals stand and how their legs are arranged. To investigate the mechanics,we present a closed optimum solution of the body foot ratio for a 2D two-leg walking machine. A multi-walker is used as a case for 3D general analysis,and the numerical simulation is presented. Both 2D and 3D case studies can explain the above observations of mammals and insects. These findings can also be used as a guide for the design of man-made limbed machines. After a discussion of this observation a definition of body foot ratio is defined which describes how animals stand and how their legs are arranged. To investigate the mechanics, we present a closed optimum solution of the body foot ratio for a 2D two-leg walking machine. A multi-walker is used as a case for 3D general analysis, and the numerical simulation is presented. Both 2D and 3D case studies can explain the above observations of mammals and insects. These findings can also be used as a guide for the design of man-made limbed machines.