Is female Wii Fit Trainer's hurtbox still smaller than the male's?

There is no competitive difference between the female and male Wii Fit Trainers in the Smash games - the differences are purely aesthetic. Their Smash 4 and Ultimate Wiki pages list no differences - they would do so if there were differences. This also applies for the genders of Villager, Robin, Corrin, and Inkling, as well as the alternate costumes of Olimar/Alph and Bowser Jr/Koopalings.

Sakurai stated that any changes between characters that are more than aesthetic and reflect different attributes warrant a separate character slot e.g. echo fighters. No difference in attributes = no separate character slot. For example the only attribute differences between Peach and Daisy in Smash Ultimate are slightly different hurtboxes during certain animations.

EDIT: As per Toomai, it's not mechanically possible for different costumes to have different hit/hurtboxes. There is only one moveset file (which contains this data) per character. Lots of other stuff can be costume-specific, but not this.