You're not wrong. Chowon's beta looked normal then got a super masc glow up when he was paired up with Chowon. I mentioned it before and people were telling me it was just a normal glow up from side character to main character. But it's clear that this is Fargo's preference. It's just disappointing that she went back to this dynamic yet again when she purposely chose to do an alpha x alpha pairing.
I kinda agree. I actually thought Byul probably just got it from his parents lol. Because looking back at the earlier chapters of Love is an Illusion, Dojin was pretty skinny for an alpha too. He only got more buff after becoming a dad.
Byul and Dojin already had a conversation about this, and Byul was aware that his physique is small than what people would normally expect from an alpha. Dojin even admitted that he used to be the same way before he met Hyesung and had Byul.
Siwoo is just built different though. He's taller than Dojin and most of the other alphas too. If enigmas existed in this verse, Siwoo would probably be one.
But yeah, I don't think the whole "Byul kept getting twinkified" is true. Because he was already a twink to begin with lol. Before he presented , everyone assumed he was going to be an omega. Then he ended up inheriting Dojin's genes and presented as an alpha, which isn't too surprising considering Dojin comes from a family of consistently dominant alphas.
Byul now still looks pretty much the same as he did before he presented, he just got taller (the only indication he's an alpha). He's still young and still in puberty though, so who knows what he'll look like when he's older.

JUST MY OPINION:: I hated love is an illusion and even though I was liking this one it really disappointed me. One of the many issues I have with this author is that they just can’t make a couple without a twink, like cmon how is byul an alpha???? he just kept getting twinkfied, the height and shoulders size difference is ridiculous