Holmes points, he’d neutralise Tunney’s ability, he wouldn’t leave openings for Tunney to take advantage.
Tunney would angle for a 10 round bout as he would know he couldn't stay away from Holmes more than 10. He might be able to stay on his bike and run away from Holmes over 10 but a 12 or 15 rounder Holmes finds and KO'S him.
Box mags at the time covered other Light Heavies comments about Mike Spinks chances of going to heavy and prevailing against Holmes. Three of the LH contenders asked reminded the interviewer that Mike Spinks walked around at about 210 lbs. and would get down to 175 lbs. So at 6'2" and 210 lb walking around weight he was big enough and smart enough to give Holmes trouble and he did.
if you check Tunney's boxrec it states he fought between 178 lbs and 192 lbs. As a stand up boxer I would not want to face a guy with superior skills and fade him 20 lbs of muscle to boot. Holmes would feel him out and then take him out quickly.
Holmes had the size, style, reach and speed to outpoint Tunney which just shows you how great he was because Tunney was as tough and smart as they come.
Spinks walked around between 187 and 192 so they were all incorrect by an embarrassing 20 odd pounds.