Minecraft wiki entry for Bundle, associated text: Bundles are used to store different item types in the same inventory slot. This does not, however, increase the total capacity of the slot: each bundle has 64 "bundle slots" and each item placed in the bundle takes up these slots similar to how they take up space in a normal inventory slot: items that stack to 64 take up 1 bundle slot, items that stack to 16 (for example, eggs) take up 4, and items that do not stack (such as tools/weapons/armor) take up the whole bundle, all 64 slots.