If you're referring to the pictures I posted of my Elezen fitting nicely, I went back later and checked the slider, and she was at 52, with 50 being the center and default. So, if we're correct in assuming the Falcon is built to the default height of each race and not scaling, then that's why my girl doesn't clip.

I think this is how it's done for all mounts that have different sizes per each race, verses the one size fits all mounts with each race/gender sitting on it with a different pose. IE the differently sized chocobos and horses verses the Ahriman or Behemoth.

It's definitely a weird QA issue, but I do think it's a height slider sweet spot deal.