What the heck does fair have to do with anything?
The difference is Hendricks is only slightly "bigger" than GSP but regularly fights at WW. Silva is noticably bigger than GSP and regularly fights at MW.
If GSP fights Silva they'll both weigh-in exactly the same the day before the fight.
By fight time, Silva would likely have a 15-20 lbs advantage over GSP.
It's neither fair nor unfair; it's simply reality. Size matters. That's why there is weight classes.
Silva's height and reach would be much bigger problems for GSP to solve than whatever weight Anderson walks into the octagon at.