Oh good grief--dimorphic size is the ancestral form! Males had already evolved to be larger. The question is whether there was sufficient selective pressure to get them to be the same size again. (Apparently no.) Humans are dimorphic in height. Neanderthals too (maybe a little less). And so on, all the way back through our evolutionary history (e.g. https://pubmed.ncbi.nlm.nih.gov/1882979/).

Only one word--estrogen? Testosterone is positively correlated to muscle growth, which is the larger sexual dimorphism anyway--height is only 10% but grip strength, for instance, is like 60%. But it also positively impacts bone growth, both in girls (e.g. https://pubmed.ncbi.nlm.nih.gov/21590732/) and boys (e.g. intervention documented in https://pubmed.ncbi.nlm.nih.gov/2001104/); and in bone maintenance in adults (https://pubmed.ncbi.nlm.nih.gov/27703340/). Yes, estrogen is very important! But it's not the only thing that's important, which was your claim.

On top of that, the logic doesn't even make sense. Of course if there's a sexual dimorphism induced by selection it has to be implemented somehow. You can't find a mechanism and go "oh, well, that explains it!--it wasn't sexual selection after all." Evolution stumbles randomly into mechanism, then preserves what works best. Better brain development AND the right level of sexual dimorphism to maximize fitness? Great! (If in fact that level of sexual dimorphism maximises fitness...and if some other level is substantially better, there is plenty of regulatory flexibility to make it happen.)

Then there's the problem of you arguing against straw men. The original argument you quote "the success of the strongest and boldest men, both in the general struggle for life and in their contests for wives" [Darwin] neither matches what you try to argue against nor does your argument even hold: "if men evolved to be bigger and stronger than women as part of a pattern of domination and mate competition no woman would be taller or stronger than the average man" [Beau]

Across primates, if males are competing for mates, they are competing with other males not the females, and secondly, if some males lose access to some females by virtue of being shorter than them (not that it works that way but let's pretend it does), it still doesn't matter to reproductive fitness as long as there are enough available smaller females so fitness is limited by other factors (e.g. sperm competition or whatever).

If anyone actually believes the bizarre hypothesis that men have to be bigger than women in order to dominate women and otherwise they won't get to mate, actually cite the argument and argue against them specifically. Don't bring Darwin into that battle.

Finally, because physical violence can be lethal or disabling, and it only takes one such event in the entire lifetime of an individual to abrogate one's fitness, none of your arguments about the implausibility of "constant fighting" are particularly relevant. Rare fighting is enough to provide an extremely strong selective pressure.

I agree with the quoted exhortations of various scientists that the development of the female body is understudied. However, the most common cell lines, HeLa, HEK293 , and WI38 are female (human), and the most common non-human is CHO (chinese hamster...ovary). I don't know what Dr. Clayton is thinking when she includes research on cells as being "mostly male"--it's patently false. It's mostly female.

I definitely agree that evolutionary hypotheses need much more rigorous testing.

But the developmental and evolutionary biology you present here is oversimplified and/or wrong to the point of being worse than nothing.