IS BIGGER BETTER?...
Everyone knows that size matters; Especially in regard to metabolism, energy costs, physiological capacities, and more. Size has meaning in nature, and according to the rules of nature. In a similar way to the financial saying "money costs money", in metabolism one can say that "energy costs energy" or "weight costs weight".
Beginning at the molecular level, size matters. Bigger molecules take longer (on a relative scale) to catabolize, and more energy as indicated by a higher volume of oxygen (as indicated by lower RER values for bigger molecules. Bigger molecules also increase the overall costs of transporting them within the body. Size does matter, since bigger molecules tend to store more energy during their anabolism (creation), and tend to release more energy per building block when catabolized (broken down).
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Size matters, since the phenomenon of the "inverted U" applies to size as well. The smaller the mammal, the greater its heart rate, and the greater it energetic consumption or metabolism. The shrew is the smallest mammal on earth (also the only one that is poisonous); with a heart rate at rest of 450 bpm, and a maximal heart rate of 950 bpm, it must eat every three hours or it will die (literally die).
On the other hand, greater mammals carry more weight, tend to be less efficient in general, and thus must consume very large amounts of calorie rich food on a regular basis, or they will die. In both examples, animals calculate the balance between energy invested to be fed, and the energy gained when fed.
Bigger mammals achieve their cardiovascular and respiratory systemic capacities via physiological volumes (stroke volume and tidal volume) and physiological rates (heart rate and respiratory rate). The assumption is that paying for every unit of rate (beats per minute and/or breaths per minute) increases overall energetic costs and shortens lifespan. Thus, increasing physiological volumes becomes a strategic goals and a tool, to decrease rates, thus prolonging life (in theory; in nature).
Size matters, since greater muscle mass costs energy. Increasing skeletal muscle mass increases the body's energetic cost in every state of existence, be it physiological rest, exercise, or maximal exercise. While this increases the chances of losing fat tissue ("losing weight"), it also means a greater caloric intake necessary for people active, exercising, and athletes, otherwise they will be in a caloric deficit.
Size definitely matters, since bigger skeletal muscles include a higher muscle fiber count (bigger muscle mass), and greater skeletal muscle force production compared to smaller muscles with less muscle fiber count. Size matters, since bigger muscle and greater muscle fiber count requires greater neural stimulus to result in force production, especially sustained muscle production.
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