While at a supercharger I once struck up a conversation with a gentleman seated in his new Model S plaid. I asked him how he liked the car. He said "yes, it's pretty quick but I dan't care for the seats". I said really? I thought they were pretty comfortable. He said he was 6' 3" and 285 lbs. He didn't look it while seated in the car.
How do you design for that. What kind of vehicle would have comfortable seats for someone that size? A large SUV?
That's a great question!
The car manufacturer just to simplify and make a seat that “most” people should be OK with in order to reduce his costs while serving the many.
Storytelling sometimes makes a point.
When I was in college, I was very fit. I was wrestling and had been for a decade. It wasn't noticeable, but I had longer arms, a larger neck and a skinny waist. I had a tall upper torso. I was and am 6'1”, which isn't very tall but it was all in my upper body.
There were no clothes for my upper body that fit!
I was forced to custom tailor clothes or wear clothes that didn't fit right.
That was true into the 1980's, I think. Years and years I put up with the lack of clothing that fit me right. As I said, years later clothing manufacturers came out with Large Tall Athletic Cut. Those fit. Finally I could buy off the rack.
I guess the point I'm making is this.
Most manufacturers focus upon the majority. And they focus on what appeals in the showroom. If a need is outside the majority, and it would cause them to spend more money for fewer people, oftentimes they won't do it.
Once there is enough of a demand the manufacturer often times will do something to accommodate.
For Tesla, I would think that the X and S could accommodate a larger body build, to a point, but chose not to. Tesla chose to use the same seat frame on all 4 models to save money and simplify manufacturing. I beleive that decision was made in the 2017 refresh for the S and X.
For many years they added a wire loop outside the frame for the X and S, and could have a slightly wider seat with fancier larger bolsters that way. But it really didn't help the larger boned person. I don't think they were trying to accommodate a larger body build.
That leaves the aftermarket, custom seat makers, and DIY'rs to do what they can.
For a really big guy, say quite large boned, quite tall, heavier, say around 6'4, say around 250+, both the Model 3 and Y are out. There isn't enough head room and side to side room to accommodate really big people.
But the X has a lot of headroom and side room to work with.
Tesla could offer a larger seat as an option but it does complicate the works and Tesla is into simplifying. I don't expect to see optional seats from the factory.
If a larger guy really wants an electric car, I think there are other companies making larger cars, possibly Lucid and possibly the Ford pickup, with presumably larger seats. If the larger guy really wants a Tesla, he would have to customize the Model X or S.
In my case, I'm “somewhat” larger and for me I can make the Model 3 work with a custom seat build. For drivers a bit smaller in frame build than me, small changes to the seat should work without a fully custom seat build.
As a note. Before I bought my Tesla Model 3, I preferred larger cars. I had many Cadillacs and trucks.
I never owned one but I've read in forums some luxury cars have adjustable width!