Since the 7.x-1.2 release the derivative back end in Islandora has significantly changed. This structure is no longer valid to create and return derivatives. See https://github.com/Islandora/islandora_solution_pack_large_image/blob/7.x/islandora_large_image.module#L304-L339 for large image's current implementation of this hook, https://github.com/Islandora/islandora/blob/7.x/islandora.api.php#L591-L661 for API documentation and https://github.com/Islandora/islandora_solution_pack_large_image/blob/7.x/includes/derivatives.inc for this in action.