It just seems weird AF that the skin packs that are ~6 MB on the 360 are 60 MB on the XB1. All the skins should be in the main game for compatibility purposes, and a mere unlock should be a hell of a lot smaller. It seems like MS threw size considerations out the window this gen, but that's nuts. Also doesn't explain why some of the story / quest DLCs are 60 MB unlocks while others almost seem like they're big enough to contain the actual content.
Though, IIRC, the original Borderlands on XB1 has the main game and 3 separate DLCs to download. The Underdome, Knoxx and Claptrap DLCs are available and, IIRC, bigger than unlocks, but there isn't a separate download listing for Zombie Island. Since the BL1 BC version includes all the DLC whether purchased separately or not, it wouldn't matter there.
I haven't played any of the Borderlands games on XB1, so I'm not sure if any of the separate DLC downloads are needed.