Why is this more impressive at smaller scale?

Put simply, smallers organisms run hotter...have a higher metabolic rate. If you've ever felt the heartbrat of a cat, chicken, or hamster youll notice that it is significantly higher than your own, thus they require more oxygen relative to their size. Interestingly, the article points out that they become someone anaerobic metabolizing fructose, which doesnt require O2, throughout the body as a source of energy instead of glucose

Many people assume metabolism is proportional to surface area based on the argument above. This idea was first proposed by Max Rubner in 1883. In the 1930’s, Max Kleiber proposed that metabolism increases according to body mass raised to the power 3/4. If metabolism were proportional to volume, the exponent would be 1. If it were proportional to surface area, the exponent would be 2/3. But Kleiber’s law says it’s somewhere in between, namely 3/4.

Its not clear which, if either, theory is correct. But both agree t