Agree this definition discussion is tiring and veered way off topic from the original questioning about Sam's future in the game.
All that talk about Trans women existing in a universe where Futanari also exist got me thinking about hormones. Female fertility medicine AFAIK, which is admittedly as much as your neighbor, is also hormones. Is it possible that those have a special interaction with Futanari specific hormones during puberty? And that is why other Futanaris might be built more like normal woman and have an easier time passing as such? If they went to that special school where they would get treatment without their consent or knowledge that could explain why Jade that got hidden from everyone since her birth passed through the net. We could easily explain Stella's physical shape if she is indeed a futanari that got through that school that way.
I think the answer is more like this.
If not mistaken her father Carlos pushed Jade to do physical training, maybe as a way to take her mind of all the bad things she kept hearing about futas on the media and also as a way to learn self defense. And she also kept going with training later in life when she joined that boxing gym.
So it would be a mix of having the genes from her parents, from being born as a futa and all the physical training she kept doing over her life so far.
Stella simply didn't had all that training so she never became a muscular femme.