They turn into their large sprites whenever I prepare them. I hope it's not intentional or just a placeholder because it ruins the immersion...

I hope this GIF helps. Notice the hotbar.

So I just looked into the issue again and it seems like it was due to incompatibility between this mod and XisumaVoid's Item Stitching Fix (resource pack) from VanillaTweaks. You may close this issue if no further fix is needed-

I didn't know that the Stitching Fix would cause that; but that is good to know :)

Not really much I can do that about that since it really has nothing to do with my mod and everything to do with the "fix"

I'm going to need you to be more specific because this is not enough information to go off of. A picture would be really helpful