Apparently Hemsworth dwarfed even normal-sized prosthetics:
“The morning we shot that sequence, we went into another room in the house we were shooting in, and Chris modeled the eight-inch and the ten-inch options for us,” said Daley. The eight-incher was quickly vetoed: “As big as it would seem, it did not look that big on him. I think it was just because Chris is such a big, hulking dude.”
“He makes the normal-size penis look small,” Goldstein concurred.