Maybe he is not as physically capable than those Chinese players.
I don't think he needs to be.
I don't think the problem is that his forehand isn't as good as top chinese players. It's that he had a fairly bad forehand for a player of that level, period.
The whole thing is just a mess.
Firstly there is so much torso rotation that I actually wonder if he sometimes has trouble keeping track of the ball, keeping your head still while your body moves that much must not be that easy. Keeping your elbow close is supposed to give you faster recovery, but if you have to arrest all the momentum of your rotating upper body, and bring it back to neutral, you really arn't gaining much (or any) time.
Secondly, there is no fluidity. Even watching the brief clips Xu Xin's forehands, you see his arm, or parts of his arm are relaxed before and during the shot. Harimoto's stroke looks like he's tensed the whole way through, even during the preparation for the shot! He's just muscling the stroke through, and he doens't have that much muscle to get away with it. There are some parts of the stroke, where it's ok to use the weight and momentum of your bat/arm. But Harimoto seems to be using, if anything at all, the momentum of his rotating upper body, this is... bizarre...
Thirdly, his feet don't stay very grounded. He has a very wide stance, so you'd think he'd be perfectly grounded. But when you watch his fast multiball, some shots, he's not only not generating power from his legs, he's actually losing power from his waist rotation into pushing his lower body the wrong way.
While, more muscle would help him muscle his stroke through more, and maybe he wouldn't need to compensate so much with waist rotation. His problem is not primarily a matter of strength, it's that his forehand stroke is biomechanically, horribly inefficient.