Why do mice have a higher metabolic rate than humans?
In endothermic and ectothermic animals, ie. warm and cold blooded animals, metabolic rates are inversely proportional to size of the organism. 


  Quoting from https://www.biog1445.org/demo/04/metabolicrate.html
  
  This is because smaller animals have a higher surface-to-volume ratio,
  thus a larger relative heat loss to the environment per unit time. To
  maintain a constant body temperature despite rapid heat loss across a
  body surface, a small animal must oxidize food at a high rate. Because
  the relative amount of food consumed and the pace of digestion,
  respiration, and so on must rise with decreasing body size, there is a
  lower limit on the size of endotherms. The smallest mammals are
  shrews, which weigh only about 4 grams. They must eat nearly their own
  body weight of food every day, and starve to death is just a few hours
  if deprived of food.


This is shows that smaller animals, like mice have a much faster metabolic rate than that of larger animals like humans or elephants.

When talking of within the cell, the mitochondrial content of a muscle cell of a mouse would be increased to produce more energy from one feeding than that of a human. This is because cells themselves increase the number of mitochondria based on the requirement of the body, thus a muscle cell of a human has more mitochondria than a neuron, since muscle cells have to utilize much more ATP to function. This phenomenon, known as mitochondrial biogenesis would be observed in mice vs human cells.
In mice, having more mitochondria would be an evolutionary advantage, as only those producing the optimal amount of energy would be able to survive the loss of energy to the surroundings and the process of natural selection would leave mice with relatively larger numbers of mitochondria in their cells.

(For more info)
https://en.wikipedia.org/wiki/Metabolic_theory_of_ecology
http://www.sciencedirect.com/science/article/pii/S0378111900005825