In The Legend of Zelda: Skyward Sword for the Wii, you can upgrade the bomb bags in the game at the Scrap Shop by collecting various items throughout the world.
Here are the bomb bag upgrades, and what you need to collect to upgrade them.
Medium Bomb Bag – Upgraded From: Small Bomb Bag, Cost: 50 Rupees, Ingredients: 1 Blue Bird Feather, 1 Lizard Tail, 3 Ornament Skulls
Large Bomb Bag – Upgraded From: Medium Bomb Bag, Cost: 100 Rupees, Ingredients: 1 Golden Skull, 2 Hornet Larvae, 4 Jelly Blobs, 3 Lizard Tails
Check out my full upgrade guide if you’re looking for the other upgrades for equipment, potions, and wallets.