Yes her sketches when she was experimenting and probably didn't know if Byul was going to be a bottom or top, but once she finalized her story it's obvious she twinkified* him to his final form now
At best he'll grow a bit more in his adult form, but he's never going to be as big as Siwoo or his dad even. The author has been consistent with her big top and little bottom physique-dynamic. That's just what she likes.
Or or or, and hear me out here.. he's his dad's son. Dojin was build like a stick until his early 20s
Also personally I dont think his so loving parents wouldve allow Byul to starve and (this is just my theory but) from what we see when he was a trainee i just think he thought he was an omega, everyone assumed he was too, considering he's a carbon copy of Hyesung. And when he presented as an alpha that destroyed the image the agency was going for and his debut was pushed back. Which it can be why he is a little iffy abt being an alpha and hates being reminded of his trainee days
My basis is on the point that the author CONSISTENTLY draws bottom twinks and large tops. This webtoon is no different from her previous works..like at all in that regards lol. It doesn't matter if Byul is an alpha, he's always going to be smaller and fragile looking in comparison to Siwoo because he's the bottom. That's why the author has placed emphasis that Siwoo is a "big guy" and it makes Byul look twinky in comparison. That's how the author likes them, and I doubt she's going to change it up anytime soon.
yeah, i don't think byul will be buffy by any mean, but i do think he'll be draw with more muscle at least on the side stories, cause fargo nim said this whole story was gonna be set during their school years from start to end. Perhaps, with a couple time skips we get to see muscke builder Byul, but honestly, i doubt he ever get to be like siwoon or Dojin.
I doubt it'll be anything drastic. It really has nothing to do with the plot but again more to do with Fargo's taste. They'll grow but Byul will definitely stay slim. Probably with lean muscles at best. He won't look as frail as an omega but he won't have that strong alpha physique either. Siwoo has to be able to tower over him in length and height basically.
it'll be good if that was treated on the plot as second gender dysmorphia because of how everyone told him he'll be an omega until last year (on the story) or explain us what happened to siwoo's family cause he seemed to be smaller and skinnier than byul up until middle school. Sadly, with how fargo handle the grandma thing, i doubt we get good plots like on the first story.

Byul isn't twinkified, he's just malnourished. What was byul training for before the story started?? BEING AN IDOL, how are male and female idols? VERY MALNOURISHED!!!. He has the right height for an alpha, when compared to OTHER alpha classmates, Siwon it's just TOO TALL. Also, Dojin was SKINNY ASF at first too, the only reason why he's buff now is because he started working out in order to look hot for Hyesung, that was revealed on their story. Was Byul allowed to build muscle as an IDOL TRAINEE? Prob not, because companies don't like BUFF idols, they like SKINNY idols, wonho and changbin suffered from that during their first years of career.