Help, looking for manga

Please help me. I am looking for a yaoi manga. I don't remember if they are the main characters or just a side story. It's about two males already in a relationship and living together. In the flash back A was the tutor for B. A is a student tutor so their age difference isn't big. Only about a year or two. In the flash back B was super small and cute like a Yorkie and A loved that. They fell in love and have a happy and healthy relationship. They have a lot of fun cosplaying. B always wore cute lingerie and girls costumes because he could fit into them and loves them. Flash forward a few years and you find out that they are still together but have been separated for a few month or a year or something (don't remember) because of work or school (don't remember) and only have been in contact through letters. A's plane touched down and he went to the apartment that he share with his lover, B. When he opened the door he did not get a happy welcome from his small and cute lover but a awkward welcome from a super tall, muscly, hot, sexy man. A was confused about why a tall, muscly, hit sexy man is inside his and B's house. The tall, muscly, hot, sexy pretended to be B's big brother. It wasn't untill night came and the tall, muscly, hot sexy man tied up A to his bed, put on B's lingerie, and started to ride A that his true identity was discovered. Turns out B had a massive growth spurt and was too scared in telling A because he was worried that A would leave him. They are still together. A says to B that he was small and cute and now he is tall, muscly, hot and sexy and he is so into it.