You fail, like all natural selectionists fail, to give a genetic why living beings grow big or shrink. I live in Florida and I've found squirrels are 30 to 40% smaller than squirrels in Illinois. You're right about the larger prey is corollary to larger humans, but you give no genetic correlation why this is. Threats and fear is the driving forces of all evolution. The threat of a large prey animal to maul a small human must of scared the hell out of those people. Over time this fear switched on a DNA molecule to make sure these humans could take down big prey.