There are currently 9 weight classes wherein the UFC employs a champion, none of which hold the title of the fastest growing division. That title, however dubious under these circumstances, belongs to the "Moneyweights"; or that abyss of fighters that seems to be forming between the 185lb and 205lb weight classes.
In years past, the UFC’s moneyweight division once consisted of only a couple of fighters such as Rich Franklin or Wanderlei Silva. They have been joined in recent months, thanks to the sustained dominance of Jon Jones and Anderson Silva, by the likes of Chael Sonnen, Lyoto Machida, Vitor Belfort. Rashad Evans, Shogun Rua and Dan Henderson seem destined to follow in the coming months. These fighters all have two things in common. They have lost to the current champion (in most cases rather convincingly) or a top contender and they are all still relevant, big names that the UFC can promote in big fights. The logic is simple; they no longer have a path to the title, so hey what not take some big fights? There is even more motivation to do so for fighters moving up in weight as they have much more to gain. For example, Chael Sonnen lost two fights to middleweight champion Anderson Silva. If he goes up to light heavyweight and wins a couple of fights, goes back down to middleweight and wins a fight, one could project that he would be right back at the front of the line for a title shot at 185lbs (most likely only if Weidman beats Silva in December but you never know with Chael). Sonnen has substantially more to gain, financially and otherwise, if he goes up in weight and fights some promotable names rather than stay and toil in the middleweight division as gatekeeper.
This is the reality facing all of these fighters mentioned; stay in their current division and become a gatekeeper as long as the current champion remains or become a moneyweight and hope one day you retain that contender status? The UFC certainly prefers the latter. It frees up matchmaking restrictions and creates fights with main event caliber fighters that would be otherwise unattainable. It is only a matter of time before each of these fighters settle into one division or the other so it is wise to capitalize when possible. But for as long as they are willing, the UFC seems to be more than happy to be in the Moneyweight business.