I 100% believe it's because the terfs feel super empowered right now because they have a popular and rich children's author on their side. Sometimes it doesn't matter how small the group is when they have all the money to tweak the laws as they see fit.
Since very few people are educated in seeing that, it continues this terrible cycle of fear and assumption.
Other comments aside, humans have at least 4 different kinds of pelvis shapes (Caldwell–Moloy classification). If you decide to look further into the research, we actually have no idea which pelvises are most common for any sex. Any research done has largely been "which best for woman give birth" and that's it.
We have no data to give a proper average of what should be expected for those assigned male at birth. We have no data for those who are intersex either but worse because even many modern practices do their best to hide everything.
No idea where you live, but to give you a start:
https://isna.org/
http://www.ukia.co.uk/
Best luck to you finding your answers.