Interesting. Where's this information from? (Not saying it's wrong, just wondering.)
Although having gone to the character creator to have a better look at the different models, your list of the 'groups of each model' may not be quite right.
To look at different characters in the same outfit, I went through the character creation process up to the "select job" screen, set it to show the character wearing their job clothes, selected Monk since it's a fairly form-fitting tunic, and then backtracked to the "select race/gender" screen, which leaves all the characters wearing that costume as I scroll around the list.
While I was doing this, I noticed that there are some differences in the designs, but they don't match up with your list.
Specifically, there are three variations of the black section around the neck under the yellow tunic:
1. A high-collared black undershirt - male Midlander, Highlander, Lalafell, Miqo'te and Au Ra
2. Collar only (like a choker) - male Elezen and Roegadyn
3. Collar and covered shoulders - all female models
Obviously it's divided beyond those three categories, but I would assume that characters wearing different styles aren't grouped together.
I think male Au Ra might need to have their own category since they're a unique shape - beyond just changing the height and width, they also have a narrow waist compared to their shoulders.
Also male and female Lalafells still get costume variations by gender (eg. female Lalafell bard has no undershirt and dragoon gets the 'stab me here' armour gaps).
Also also, there's headgear, particularly helmets, which needs to be shaped differently for Hyuran vs Miqo'te and Elezen vs not-pointy-eared people. (I'm not sure how HW/SB gear handles Au Ra horns, but ARR relic gear clearly doesn't care - it just clips straight through.)
So if they do use one model for multiple characters, perhaps different types of gear are divided up on a piece-by-piece basis rather than grouped the same way every time.
I've seen people bring this up a few times (usually that everything except the race-specific starter gear ignores the slider setting) but I don't believe it to be the case. Characters can have noticably different bust sizes with the same gear equipped - and by equipping job gear in the character creator and backtracking to the body settings, it's clear that the clothing shape changes as you adjust the slider.
If anything, this is why they won't implement a 'butt slider' - not because it wouldn't do anything, but because to make it do anything they would have to go back and remodel every single 'leg equipment' item in the entire game to add new adjustable parts to the shape. (And most likely need to check that it won’t clip through shirts or coats.)