It's typically weight and balance, e.g. where the centre of weight is. E.g. a typical cessna 182 could carry 4x normal adults. But if you chuck a single fattie in the rear storage compartment, that plane ain't going anywhere. The pilot was just doing his job, FAA regs will state max pax weight is as listed by manufacturer, in this case, 300lbs, any more and you're not allowed. Or guess what happens when it has a hard landing and an investigation is launched: the pilot gets shafted.