a switch? idk pls help me remember it

Here me out, it was a bl manga I read a long time ago and I remember they were already in a relationship at the beginning of the manga but he is getting insecure(?) because ML is getting bigger and bigger since idk also I forgot too- maybe puberty or the like. He was seme at first in their relationship but because their height and body differences is getting more noticeable, he was getting worried about that

at the end, their roles reversed and now he's uke while his partner is seme