This make me remember some oneshoot that also has same vibe. They compete each other since kid and then they marry and ask their kids who the kid love more .
You're talking about the yandere oneshot.
Where the yandere was all intense but the guy was still enthusiastic and up for it and just swept up the yandere in a normal relationship
She definitely good a breast size reduction from middle to end.
But before that she also miraculously got a breast size enlargement between the start and the middle.
Compare her chest in the following panels:
1. Where she says: "Nope, not a one." --> Frontal view of her: Her shoulders are wider than her chest.
2. Where the guy says "The game of real life." --> Frontal view of her: Her chest is wider than her shoulders. = Chest enlarged
3. The foto in the last panel where she holds her baby. --> Frontal view of her: Her shoulders are wider than her chest again. = Chest reduced to original size.