At 00:48, video of the Japanese Spider Crab, or the Dead Man’s Crab*, occurs and touted as the world’s largest. But is it? Sure its big with a leg span that can reach several meters but it is all leg. Weight or mass wise, I would argue that the world’s largest crab and modern arthropod is the Tasmanian Giant Crab.
*Called because its been seen scavenging on the bodies of the drowned