Height does not determine dominance.

I agree if we were just talking height alone, but there's been chapters where Seto has been shown to have her limits, and she's more than capable of being assertive with both Sasaki and Nanasawa at the same time. And not only that, but Sasaki's meddling is not only changing her younger self, but Seto now too. After she works out her feelings, I wouldn't at all be surprised if she wanted to turn the tables on Sasaki at least once. She's definitely got the personality for it. It's just that it's only now coming to the surface.

I don't think it's been brought up yet, but Seto is still taller at 16 than Sasaki is as a full grown woman. Now that her switch has been flipped, what's going to happen if she decides to start making some moves of her own? If she wanted, she could easily be the dominant one in this relationship, to say nothing of Nanasawa.