Years ago, I'm pretty sure I read a manga featuring a non-human seme, who possibly ate humans??, and a human uke. I think he might eventually have turned into the same kind of being, and was bigger than the seme? He might also have gotten pregnant at some point.
There was also a secondary couple, non-humans both, and the top was absolutely shorter than the bottom. They were trying to have a kid, I think.
My brain keeps insisting it's by Ike Reibun, but I can't find it in her bibliography and I'm not even sure this manga actually exists, and isn't just a fever dream.