more info plz, ill b sure to help!! I have 3 that may be it but not sure :D
maybe this?
https://www.mangago.zone/read-manga/love_gym/
It was a manga and I do remember the characters having a love session at the entrance of the door. But the mc was still chubby with doing it
mmm well i found this one tht fits ur desscription,,,
https://www.mangago.zone/read-manga/shinnyuu_shain_no_omega_desu_ga_senpai_alpha_kaizoushimasu/
I HOPE ITS THIS ONW :)))
OMG YES, ITS THIS ONE! I thought I wouldn't be able to find, let alone read it. Tysm!
NO PROBB! IM HAPPY I HELPEDD. i might read this myself :D
All I remember is the guy starts out fat and loses wait with the ml