Responses
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
i don't really understand the uke age and since the person in the first chapter was calling him "kid" this is weird; still his body could be due to malnutrition but still weird weird