I mean, they're same age and the author gave a good reason for it (lack of nutrition)
I get what you’re saying but normally even if they’re the same age, if there’s a noticeable size and maturity difference between the mc and ml it will give me the ick. However that’s not the case for me at all with this one
I'm guessing its because despite the difference in both size and maturity, they both treat each other as peers and you can see that seong is comfortable being spoiled/lax/showing his childish side only with jeonsu instead of only trying to act mature. Oh, he's also nurturing and caring despite being a jealous guy
I normally dislike it when there’s a major size difference, but this one is just so endearing and cute