Why Mai from the Future Trunks timeline is younger than what she should've been?
In the future Trunks timeline the Dragon Balls have been destroyed, during the Android Saga, the Pilaf Gang should not be able to wish for youth.
Her appearance in the future timeline, appears to be younger than what she should've been at that age. She even appears to be younger than what she was in Dragon Ball.
Pilaf gang made the wish just before Piccolo were killed by the androids.
Toriyama's Dragon Ball manga and both Dragon Ball Z and Dragon Ball Super anime adaptations didn't cover that event IIRC, but Toyotaro did in a special chapter of his manga (in Vol. 2). According to Dragon Ball Wiki:
In the future timeline, Future Android 17 and Future Android 18 are battling the future Dragon Team, and Future Goku is dead - having died from the Heart Virus. Future Bulma and Future Gohan are flying in a capsule ship and notice the Dragon Balls have been activated. Future Pilaf wishes for himself and the rest of his gang to be reverted to their youth - as babies, and Future Bulma arrives too late to stop them from making a wish. The Dragon Balls then drop down as stone, as Future Piccolo has died. Future Gohan screams in anguish at Future Piccolo's death. The baby Future Trunks and the baby Future Mai notice each other as baby Future Pilaf plans to escape.
It's assumed that something similar happened offscreen in the main timeline.
