“As the new century began in UC 0100, the minitype Mobile Suit revolutionized battlefield situation. This Miniature type of Mobile Suit would be called the fifth generation MS as it achieved the dual objective of miniaturization and high performance due to use of a new type of a nuclear fusion furnace.
In addition, the performance of fifth generation Mobile Suits would go far and above those developed before UC 0090. These became known as ‘phase II MS, with an average height of 15 meters, they were invested and developed by Anaheim Electronics and the Buch Concern–the latter of which would become the Ronah Family and funded the Crossbone Vanguard.”
-Gundam Fact File #91; “History of Mobile Suit Development, Part 21”