I think it relates to the surface area:volume ratio. The implication is that a smaller animal will have a relatively higher surface area to lose heat to, thus n

I think it relates to the surface area:volume ratio. The implication is that a smaller animal will have a relatively higher surface area to lose heat to, thus necessitating increased metabolic rate to make up for that loss. http://www.tiem.utk.edu/~gross/bioed/bealsmodules/area_volume.html http://www.tiem.utk.edu/~gross/bioed/bealsmodules/area_volum... provides some good basic info