@Sigilbreaker26
Misasa’s size is an advantage, though. It means he doesn’t need as much space for his moves (no large windups or a need to spread his body out; see Yumi’s stance at the end for his iai move with his right leg taking a large step forward and to the side). Combined with the flashback that showed how he was able to smash a dent into that rock with minimal movement, that means he doesn’t have as large an issue as Yumi has in picking out what moves to make as the ring shrinks.
Yumi, on the other hand, has to be careful with what moves he makes. It’s not a problem if a part of his body is out? Sure, but that means he’s at the edge of the constantly shrinking ring instead of moving inwards and keeping himself safe while pushing Misasa out; all Misasa would need to do is push out the parts of him still inside (that one foot) the shrinking ring or keep Yumi from moving from the spot and the match goes to him. What’s important here is the fighter’s ability to fight in a small area. There’s also Yumi’s longer limbs that make it more inconvenient for him to defend himself when they’re real up close, which is why Misasa’s smaller size is an advantage here.
If the ring gets small enough that they’re face to face and unable to move from the spot, then the advantage is completely on Misasa’s side, given his stated specialty in fighting real close up. I’m not saying your point of Yumi only needing to keep one foot in is wrong, though. More just that what I’m saying here is all relating to what’s been stated by Lolong in this chapter on the psychological walls Yumi’s been trapped in. As long as he can get over that, he can get back his earlier advantage in the fight.
TLDR; you’re ignoring the fact that Yumi’s moves are becoming more restrained because he’s more preoccupied with staying safe instead of using his size advantage to its fullest. If he becomes more willing to take on risks, he should be able to equalize things a bit.