We officially have another "Too Big" situation here.
The moment he said "for good boys and girls, man, RIGHT IN THE FEELS.
Makes me cringe at Morrigan...
Define "Too Big". Cause if this is about Mai's bonkhonagahoogs, that may not be the case and more of a case of CERO being strict with character models that have upskirt shots.
Because while English translated it into a "funny gag line" due to CERO not being a thing in the West, in the JP version, Sakurai mentions that CERO gave him issues regarding Mai and couldn't get her in unfortunately. Sakurai, after Smash 4, also mentioned CERO giving him issues during Smash 4, regarding Palutena, who is far tamer than Mai, regarding the slit in her dress and they had to stitch up the slit quite a bit before CERO allowed them to keep the Everyone rating.
So very likely, it has nothing to do with Mai's breasts(especially if they gave her a tame pose that didn't flaunt them) and it may have more to do with CERO being ridiculously picky with Mai due to being able to look up her lower region, even if it was impossible to see that region during gameplay unless people directly looked for a way.
That's likely the reason why Samus, Pyra and Bayonetta(after being toned down) are left alone despite Samus' alternate outfit, Pyra's skin and Bayonetta's sexy poses. Because compared to Palutena, Corrin, Mythra and Mai, shorts/suits/etc seem to be acceptable compared to gear that allows panty shots.
So Morrigan may still be possible since Morrigan can't be up-skirted, due to wearing a leotard with leggings. It depends on whether or not CERO does have issues with Morrigan's badonkers, but Morrigan's cleavage could be covered up enough that there's no drastic design changes.