Being a dominant omega has nothing to do with their build, Grayson just got the extra large dominant omega version.lol
By the way in the KMIYC novel Chase had less musclemass than Josh, a normal omega, the manhwa made Chase too buff.
But they made Koi a touch too twinky, yes. He is not twinky like Yeonwoo from KML. He was like that in highschool, but after the timeskip he is 1,80m tall and does have some muscles, but is still far away from someone who has a bodybuild like Dane.
Well , I thought if a dominant alpha has monster like builds , dominant omega should be built or drawn stronger than twink omegas too. That's what my logic says , that's all. Well , Koi , looks like Kim Euihyun from Low tide in twilight , so don't really mind. Jig , the Author of this novel , should have made Koi look buffy ... It would have looked hotter compared to how big Ashley Miller ( Grayson's father ) is !
No, all omegas can get pregnant in this universe.
You don't really have recessive here.
You have scondary genders that were mutated to omegas. For example Yeonwoo who was a beta before. Who has some health issues, so his pregnancies (two) were problematic. Also the later one is mutated the more problems.
Anonther one is Dylan Avery.
And Ashley Juliet who was a gamma before. There the chance for surviving a mutation is already low, then having weak health which even worsen's with childbirth (one).
Then you have normal omegas like Josh, who already has three children. Or Yujin who has two children.
And then the extremely rare dominant omegas like Dane, Angel, Koi, Bliss, and Jhon.
Those can cause pheromone shock on dominant alphas or force them into rut. And have some other abilities like they recognize fellow dominant omegas, Koi helps Ashley health with his pheromones and later helped Dane. And it seems like they can mutate others to alphas.

I can't believe they drew Grayson's mother look like a twink lmao ! He's a dominant Omega , should have made him looked stronger than Joshua