The Powder Keg found in Majora's Mask is a much stronger version of the bomb. It is used mainly to clear the path to the Romani Ranch on Day One or Day Two, It's also used to blow a hole in Ikana Castle. It can only be obtained and carried when Link is in his Goron form and after he has received Powder Keg Certification from Medigoron.
Powder Keg Certification
The Powder Keg Certification allows you to use and buy Powder Kegs. After you defeat Goht and Snowhead experiences spring again, melt the ice in front of the Goron Powder Keg Shop. Go in and speak to Medigoron. He will tell you that before you get your certification, you must first pass a test. He gives you a Powder Keg and tells you to set it by the boulder in front of the Goron Race Track before it blows up. If you are successful, go back and speak to him again to receive your Power Keg Certification. This will stay with you when you go back to the first day, so you can buy Powder Kegs from the Goron in the Bomb Shop.