I didn't even know this pack was on Steam! If that is through RPGMaker, then it is likely that they can have larger uploads, whereas here on Itch the uploads have a relatively low limit and thus require more zip files.
Yeah, this pack is sold on Steam as an RPG Maker DLC. Once installed, it shows up as 1.63 GB, which is noticeably smaller than the combined size of the ZIPs on itch. Also, looking through the asset contents, it seems to be missing the ground assets shown in the screenshots. Could a part of the content have been missed in the Steam upload?