When using an anvil and placing an oversized stack in the second slot, the entire second slot will be consumed even when unnecessary to the operation. For example, combining an armor item with an oversized enchanted book stack will consume all of the enchantment books, even though a single item from that stack could've been used to achieve the same effect.

I would expect the anvil to leave the remaining items in a stack minus the one needed to perform the anvil operation in relevant situations. For example, 99 enchanted books would become 98 enchanted books with the enchantments from one of those enchanted books being moved to the result stack.

Steps to reproduce

1. Place down an anvil and obtain an armor item.
2. Obtain an oversized enchanted book: /give @s minecraft:enchanted_book[max_stack_size=99,stored_enchantments=\{mending:1}] 99
3. In the anvil, place the armor into the first slot and enchanted book stack into the second slot.
4. Perform the anvil operation by taking the enchanted armor out of the result slot.
5. Observe that the enchanted book stack is now gone rather than having a count of 98.