The justification that will often be offered is that allowing more and more body-types requires more and more work making sure every armor, cosmetic, outfit, hairstyle and visual effect maps correctly on to the body of every character and NPC that needs to wear them.
And that's true, you can't just type "add_bodyslider=true" in the code and be done with it, every seemingly simple feature in game development requires ten times the design and QA work you'd expect to make it work. Adding larger bodies to the game would have taken resources and production time, which would result in some other features of the game being less developed, or scrapped entirely. This is true of all parts of game development: there is always a tradeoff, and a prioritization has to take place.
But that also reveals the values at the heart of the production. Doing the work and spending the resources it would take to allow fat people to even exist in their world was not worth it to Larian. It could not be prioritized above any other desirable feature, everything else in the game mattered more than this. THAT is a statement of values, and it is kind of an ugly one. The right for fat bodies to even exist in their world is, at the very best, a frivolous luxury that can be safely ignored.
The company knows full well that society is largely fatphobic, and that it can easily get away with excluding fat bodies, that it can rely on the baseline hateful bigotry towards fat bodies to smokescreen them from criticism. When a game like Baldur's Gate 3 launches with such limited selection of body-types, they know there will be no major backlash, and anyone who does criticize it will be shouted down by the worst people on the internet.
But this prioritization is a straight up gaping flaw in the game as an RPG. One of the central pillars of RPGs as a genre is character creation. Character creation is arguably the actual HEART of Dungeons and Dragons, one of the most important and compelling things that a player gets to do... and Larian made that pillar deliberately worse by excluding some of THE MOST COMMON BODIES IN THE WORLD from being legitimate forms of self-expression. Baldur's Gate 3 is a game which pretends that those bodies don't exist, pretends that nobody wants to tell stories about them, and simply assumes that everyone else will play along with that.
Final Fantasy XIV, another game which I love, is another bad offender in this regard. If you want a chance at a substantially different body type for your character, you straight up have to change species, and even then, your only options are jacked, slim or smol chibi bean. It is a game which is otherwise so radically expansive with its options for self-expression through fashion, but it refuses to allow players the freedom to do anything less than adhere to a small selection of beauty-ideals when it comes to their bodies.
And again, adding this kind of diversity to a game isn't easy, but the fact that body diversity is not valued, is not prioritized, is not worked on, leads to knock-on effects where the institutional knowledge and tools and standards needed to make creating that kind of diversity easier simply do not get developed because nobody sinks time into the process. And every time a company decides that body-diversity in character creation is a frivolous, unnecessary luxury, it becomes easier for the next company to make the same decision since that's just "normal."
Larian probably feels like they had good reasons to limit character creation as much as they did. They probably feel like the many other excellent features of the game more than make up for it, and the sales figures will seemingly back them up on that.
But the lack of fat bodies in Baldur's Gate 3 - and indeed, ANY kind of body that isn't skinny or buff - is a big flaw in the game, it is in fact a gaping failure punched right through the core of the game, covered over with newspaper and painter's tape, protected by an unspoken social agreement to simply pretend it doesn't exist and isn't a problem.
