Size estimates for extinct creatures are usually based on proportional relationships extrapolated from living or more complete relatives, but there are limits to this. The true size of gigantopithecus is actually very uncertain and estimares often get revised upwards of downwards, considering the animal is basically unknown apart from teeth and a few jawbones, it's closest modern relative (the orangutan) is not that closely related, and it almost certainly had large molars relative to it's overall size. It was certainly big though, at least as large as a modern gorilla based on the absolute size of the jawbone and the minimum size of the whole skull needed to accomodate the teeth.