You can TOTALLY see the difference in technique each of them uses in this gif. 

Yuuri tenses his core, tightens his legs and keeps his posture. His back is straight, he’s curling up  and clearly using his abs/core to pull himself up. He’s doing it right. 

Yuri, on the other hand, is using his neck and arms to pull himself up, arching his back and relying on the hook of his legs in the rack to get himself started. His posture’s all wrong, he’s not activating his core, and I bet you his neck is KILLING him after he’s done with these. 

In his determination to keep up with Katsuki, he’s thrown all Form to the wind. 

It’s PLAIN to see the difference in body-types too. Willowy, thin Yuri and thick, tightly muscled Yuuri.