I suppose siyoo is just extra tall and build cus he had to do physical type of work to earn money while byul was wealthy family's treasured baby
It's either cuz byul will turn out to be button but honestly it's probably why the reason they're so drastically different but if you look at byul with the others he's pretty tall and on the lean mascular side I assume
Siwoo is just extra tall and broad
Byul himself is tall and has broad shoulders compare to his classmates
I think the artist should make Byul's build like his father. He looks closer to a twink in front of Siwoo.
I think you are forgetting a key detail, he was a k-pop idol trainee. He is used to a certain lifestyle; exercise, diet, etc. meant to keep a slim figure, even if he is tall and is predisposed to be muscular.
It makes sense story wise, but as usual for this and many authors, they tend to make one person more feminine and petite. So the latter, may be the reason why.
I don't understand why byul and siwoo's build is so drastically different when they're both Dominant Alphas.