Tsubaki is sadly not drinking milk diligently, he's growing at a snail pace.
It's just unrealistic
Theyre 7 years apart? 10yo is that small so normal, just wait for the timeskip 17yo tsubaki will be taller than yuu-nii
But he looks like a toddler His head reaches the bottom of Yuu's hands when they're walking
Sometimes mangaka are quite lax with accurate height differences lol
I think he would realistically reach yuu’s chest? Idk if we’ve gotten any canon heights for them tho
I just went back and looked at chapter 128 and it’s just the perspective of the panel that makes him look that short. In previous chapters we can see that tsubaki reaches about midway on Yuu’s stomach
It annoys me how short Tsubaki is in comparison to Yuu like it shows in 128th chapter when they hold hands like bro they're not THAT apart in age for Tsubaki to be that much shorter???