Useful Backpacks adds three tiers of inventory expansion in the form of backpacks. The small pack adds just a minor increase to your inventory and it’s accessed simply by right-clicking it in your hand.
And then a much larger upgrade is the large pack.
These packs are like carrying portable chests with you to maximize the somewhat lacking inventory in standard minecraft. This mod also allows you to craft the packs in any color wool the game comes with so you can coordinate!