If it is a 'legacy fight', they meet in the middle at catchweight 165 Ibs. Otherwise Khabib steps up and tests himself at 170 where he has fought before. Considering Khabib literally almost died trying to make 155 shows it is unnatural and totally unfair to force GSPs body to try it for the first time. If he is a true FIGHTER he fights GSP where GSP is a legend at 170. He already has the age advantage he wants every advantage he can get. I see no reason for GSP to challenge a guy in a lower weightclass he never fought in before.