In the course of 20 years of hiking, Michael O’Shea, a physicist at Kansas State University, Manhattan, noticed something rather peculiar: Smaller individuals often seemed better able to carry a heavy pack than larger ones of equal fitness. Not only was his observation counterintuitive, but it also flew in the face of traditional backpacking dogma, which prescribes that a pack’s weight be proportional to the hiker’s weight. But this seemingly logical premise failed to take into account a key variable: Bigger people must carry their own weight as well. Sure, an elephant can uproot a tree, but anyone who has ever watched an ant pick up a potato chip can attest to the fact that smaller animals have the highest strength-to-weight ratios. According to O’Shea’s model, published today in The Physics Teacher, it’s that ratio that allows lighter hikers to carry heavier packs. A larger human may be able to carry more weight in total thanks to their increased muscle mass, but the extra power from those big muscles isn’t worth their added weight. According to the model, a 22-kg pack for a 50-kg hiker will feel like a 15-kg pack for a 110-kg hiker.