>>44351990I think she just started wearing a sports bra and it compressed her sweater puppets to a smaller size. Well endowed women can find running, jumping and sports uncomfortable in a standard bra due to bouncing. May always had a sport outfit and looked athletic, so I think her melons bounced a lot, so she looked into compression or sports bras to keep them contained.
that's my headcanon reason. I think the animators were asked to size her down. 