You have a lot of questions rolled up into one, and we will have to unpack them.

To get a sensible answer, we have to consider the problem on a per cell basis.

Smaller organisms have fewer cells, but these cells are more or less the same size as those in larger organisms.

Graph 1 illustrates the relationship between the relative size of a particular type of neuron in the brains of different mammals. A 10,000 to 100,000-fold change in the volume of the organism, for example between a mouse and a whale, leads to only a 2 to 3-fold change in neuron size (note the differences in axes scales - the vertical axis is logarithmic, while the horizontal axis is linear).

Because of this, we can assume that there is a relationship between body size and total cell number.

In a similar way (graph 2) there is relationship between overall energy used in watts (metabolism) and body size (both scales are logarithmic).

At the cellular levels, a major use of energy is to maintain the cellular environment. Molecules must be moved across the cell's boundary, the plasma membrane, and this movement often this requires energy. Since such processes are not 100% efficient, they generate waste heat (this is one of the laws of thermodynamics).

Generated heat and waste products must in turn be exported. Both import and export processes are limited by the amount of surface area available.

Large animals can stay warm longer because the rate that their metabolic heat escapes is limited by their surface area.

Smaller animals must eat relatively more, and so have a higher per cell metabolic rate, because heat escapes their bodies more efficiently. Amazingly enough, animal size and metabolic rate correlates with rates of mutation and evolution...