From what I think, it's probably just malnutrition since he did said that it was his first time having such meal and plus on his past life he is treated badly and he is skin and bones. Also, he was transported to another world which probably why the seme is huge cause he is not an actual human, I think he is an elf, I can see his ears but the size is probably normal in that world the MC was transported in and his health is probably bad that's why he is small and so thin.
They will divorce in chapter 101. I'll try to find the novel, Chinese though, it's already finished you can just translate it.
Lol here https://m.shubaow.net/168/168663/
Honestly just read Boys Love (BL) but this story is really cute and I didn't come to hate it. I've really loved this, I can't wait for the next chapters to see their baby ヾ(❀╹◡╹)ノ~