Goliath beetle larvae reach masses reported at up to roughly 100 grams, a figure that places them among the heaviest of all insects. The adults that emerge are lighter, commonly in the range of tens of grams, because metamorphosis reorganises larval tissue and loses water and stored material rather than accumulating more.
The larval diet explains part of the size. Unlike many flower chafers whose grubs live on rotting wood alone, Goliathus larvae require substantial animal protein and grow rapidly on it, a requirement discovered by breeders who found wood-only diets produced undersized adults. Development takes months in soil and decaying vegetation before a hard earthen pupal cell is formed.
The common ranking of goliath beetles as the world's heaviest insect is not settled. Several giant weta, some stick insects and certain other scarabs have gravid females that rival or exceed these masses, and comparisons across life stages and sexes rarely use a consistent measure.