Although it seems logical, "surface area to volume ratio differences" is probably NOT the explanation for the fact that large animals have smaller weight-specific metabolic rates than small animals. Why can't it be the explanation?
1) Surface area to volume ratios get larger as animals get larger, rather than smaller as weight-specific metabolic rates do
2) Animals do not really lose the majority of heat across their surfaces, but mainly over any un-insulated areas (feet, ears, etc.)
3) Animals can change their posture to limit surface area exposure to the environment, effectively cancelling size differences in surface area:volume ratio
4) It only makes sense as an explanation for endotherms, but ectotherms have the same relationship between surface area and volume as endotherms do