The extinct largest primate

Gigantopithecus is the extinct primate whose existence is confirmed based on only three fossils. Gigantopithecus is said to be the largest primate in history. It seems to be the near relation of Homo sapiens than the orangutan, not than the gorilla. The size of Gigantopithecus is supposed to be large by the enormousness of mandible. I wonder whether actually it was so large. It may be an imagination of a scientist with a heart like a boy who thought the awful size interesting. It may be a normal sized gorilla-like animal whose face was abnormally big. Gigantopithecus was considered to disappear by losing in the competition with the panda and Homo erectus.

Why do the extinct animals look like so wonderful?