There are studies that show men and women have different brain sizes

You follow this by linking to an article whose title is:

Massive study reveals few differences between men's and women's brains - Neuroscientists conduct meta-synthesis of three decades of research

And which starts out with this:

How different are men and women's brains? The question has been explored for decades, but a new study led by Rosalind Franklin University neuroscientist Lise Eliot is the first to coalesce this wide-ranging research into a single mega-synthesis. And the answer is: hardly at all.

https://www.sciencedaily.com/releases/2021/03/210325115316.htm

The article in the second link doesn't give enough detail to be able to make heads or tails of it. For example, it doesn't say how big the study it's describing was, meaning how many persons' brain scans were compared. Moreover, all the adolescents who were identified as trans were on GnRH agonists (development blockers). Finally, the results of the study hadn't been published & subjected to peer review at the time - the article was just about a a presentation given at a conference. It's not clear if the paper has been published & peer reviewed yet. I have done a search but can't find it. If you have a link, please provide it.

Here's the link to the description of the study presentation from the conference: https://www.ese-hormones.org/media/1506/transgender-brains-are-more-like-their-desired-gender-from-an-early-age.pdf

Someone on Mumsnet says the abstract says.

"In conclusion, we found that boys with GD overall showed neither a typically male – nor female pattern of white matter microstructure, whereas girls with GD predominantly had sex-typical white matter diffusion characteristics with only slight masculinization in fiber organization. Our findings provide new preliminary evidence for the hypothesis that natal boys with GD may have undergone atypical neuronal sexual differentiation and possess certain neurobiological characteristics of their experienced gender."

But the poster didn't give a link. Someone else noted on Mumsnet:

This study found no difference in brains of pre-teens who identified as trans compared to those who did not. But did find there was a measurable difference in the brains of adolescents who were taking the drug triptorelin and those who were not. The abstract ascribes this to the fact those taking triptorelin identified as trans not because they were taking triptorelin - a drug known to impact the brain. Indeed, it is strangely silent on the fact the ‘trans’ participants were taking anything at all...

https://www.mumsnet.com/Talk/womens_rights/3534028-brain-scans-show-differences-in-trans-youth

There's more in the Mumsnet threat. I'm saying they're correct. I have no idea. I just don't think things are as B&W as you think.

Also, many people on your side, including influential experts at WPATH, pooh-poohed the study. Here's just one example:

https://thinkprogress.org/transgender-brain-scan-study-68f9ba4b1c43/