A sub story/ship in Never Understand (starts at chapter 55). Hot popular seme falls for pretty uke. Popular guy didn't recognize the guy he fell for was the chubby kid he insulted and called names before his (uke glow up).
https://www.mangago.zone/read-manga/never_understand/
Renai-rubi no Tadashii Furikata
Not exactly bullying, but there was a rejection in their school years, a glow up, and finally a reunion in No Love Zone and The Third Ending.
The seme in No Love Zone was bitter about the rejection, but the uke in The Third Ending was simply cautious.
Check this one ヾ(❀╹◡╹)ﾉ~
https://www.mangago.zone/read-manga/a_shoulder_to_cry_on/
Can someone recommend a story where the male lead is bullied but then has a glow up and the bully and victim start having feeling for each other? my bad if that was a bad explanation