Why are some prophets in the Bible called minor prophets?
There are twelve books in the Bible known as the minor prophets. The difference between the major and minor prophets is not an issue of importance, but rather the length of material written. The twelve books are all in the Old Testament consisting of Hosea, Joel, Amos Obadiah, Jonah, Micah, Nahum, Habakkuk, Zephaniah, Haggai, Zechariah, Malachi.
An interesting fact to keep in mind is that when the minor prophets were writing (around the seventh to the fifth centuries before Christ), the promise God had made to David that there would be an everlasting kingdom -- that promise was already three hundred years or older. These wonderful prophets foresaw horrific destruction and sorrow for God's chosen people and also for the nations that would not honor the Him. Even with all the chaos that they had total faith that the promised Deliverer would come for His people and the nations that would exalt Him.