CB makes a good point, moving up in MMA is such a bigger task then in boxing, guys go up 3, 4 pounds and get another belt, its 15 and more in MMA, being pushed to move shouldn't be as often like in boxing, guys generally in MMA move down which is somewhat different then boxing, more wrestlers, shorter rounds, etc...
If Silva wants this fight, he can make 170, just like Cyborg wanting Rousey, you know the class, go make it