Responses
Logical:
The ass is more elastic than you think... Ever heard of fisting? It can fit, with practice.. A lot of practice. He should've been bleeding since they went at it with little practice considering it takes weeks to heal and get used to, but then leads to the second answer.
Fantasy:
Our big boi, Caius used magic to numb the pain and made Koichi more elastic and sensitive. Why? Magic.
I'm sorry but can someone pleaseeeee explain to me how THAT size fit into his tiny hole? Please anyone? Explain so my confused brain can comprehend HOW IN THE WORLD THAT IS POSSIBLE.