Alisaie is an adolescent. When she fills out a bit and grows up, her head will be as tiny as that of any other Elezen (presumably). There are lots of young Elezen in the game who haven't had their growth spurt yet so their heads look relatively large.

The answer to pretty much everything else is 'fantasy'. They didn't call us 'elezen' and 'hyur' because they couldn't spell the traditional words, they did it to allow them to deviate from established 'rules' and make their own twists on fantasy staples. I'm ok with the idea of a race without so many physically differences between the sexes; female Elezen are more graceful while male Elezen are more wiry, but both having broad shoulders, big hands, fierce expressions...? Why not? This is a world with a race where the hyper-feminine women are half the size of the hyper-masculine men, and with another race where the women and men look completely indistinguishable to outsiders. And if we must include real world examples, there are plenty of unusually tall women or unusually small men even though we only have one playable race IRL (humans). Diverse physiques are cool. Clinging to a narrow idea of 'normal' is less cool.

I would struggle to main a character of any race but Elezen because I love them so much the way they already are. There's really no good alternative on offer, so please don't change us.