Which factor preventing the mouse from reaching to size of an elephant

Mammals are found in different sizes in nature. Most of scientists believe that environmental circumstances and mammals' living type determine their body size. So scientists haven't been looking forward to find a physiological factor for this size difference among mammals. But my research on animals size has specified that there is a direct relationship between animals size and circulatory system. The mouse and the elephant are both the mammal. Both have 4-chambered heart and all peripheral conditions such as gravity and atmosphere pressure are same for both of them today. Why elephant can get great, but mouse can't? Which physiological factor is causing this? I've discovered this factor. Why mammalian metabolic rate have inverse relationship with the size of the mammal? I've discovered a real factor for this question. We can use this science in applied sciences (like husbandry and nurturing domestic animals) and maximize or miniaturize body size of domestic animals due to necessities of poulterer. Of course human has done this work previously unconsciously on domestic dogs. Human has created dogs in different sizes artificially without any awareness of mechanism of this action.