Personally, I think it's barely an advantage, and, if you believe the guy tried to lose weight, then he'll be weaker, plus slightly demotivated for losing a cut of his purse already. But that's my non-MMA brain talking.
As far as most matches are concerned, either boxing, karate, judo, MMA, UFC, etc. the one that always has the advantage is those who gained weight during training compare to trying to catch weight by reducing or diet.
Referring to Zhu Rong, a bit of over 2-5 pounds shouldn't be a problem because after all in the actual match, they are weighing about over 10 pounds.