To be completely honest, while it is true they don't receive the same treatment by the narrative nor the characters around them, I gotta admit I don't think they were actually pitted against each other.
Though there is no denying Eva was treated as this irresistible, perfect waifu while Amalia is either ignored or mocked.
I will never forgive how Amalia was essentially season 4's deuteragonist and she only got a comment about her feet in the artbook. Whereas Eva's arc was fairly minor and contributed nothing to the overarching plot, and yet, her artbook pages were filled with comments from the crew gushing about her and about how "pure and strong and whatnot" she is.
Having said that, if you watch their interactions, Amalia isn't really resentful of Eva just because she's more popular. If anything, she's more annoyed than anything that everyone focuses so much on her when she's the princess. And the thing is, she is right.
Attractiveness aside (in which case I'm glad a literal 13-year-old wasn't constantly lusted after by a bunch of adults, not like Eva deserved it either), Amalia is royalty while Eva is just a random Cra. For all intents and purposes, she should have been targeted more than Eva because her life is that much more "valuable."
The fact that there's so many episodes where people take an interest in Eva or try to kidnap her, yet there's not a single one where Amalia is taken hostage with the intention of asking her kingdom for a ransom already says a lot about how little thought was put into the implications behind these characters' backgrounds.
I mean, why introduce Eva as Amalia's bodyguard as well as her lady-in-waiting, if she's never going to be put in a situation where she needs protection?
(Again, we're talking about deliberate attempts on Amalia's life as a Sadida princess. Adventuring notwithstanding.)
Also, while I do appreciate Eva and Amalia didn't stoop to petty, catty arguments over who's prettier, I gotta admit they still missed a golden opportunity to provide a wider exploration of their dynamic.
For Amalia, it would have added layers to her character if she'd grown insecure over her looks after years of being overlooked in favour of her older, more developed best friend. With the strain that could have put in their relationship forcing Eva to make a genuine attempt at reevaluating their friendship and putting in the effort to heal their bond by reaching out to Amalia and trying to fix things. You know, to allow her to do something that doesn't immediately tie back to Tristepin.
(Oh, Cleophée Arc, my beloathed).
Now that you mention their designs, while I must admit I'm not too bothered by them (other than Eva's suit from season 2 being a bit of a thirst trap and her breasts a bit too anime-like for my liking), your comments alongside @saturnyukaa 's do make me realise it would have been better if Amalia looked more her age. It's okay if she's taller than Yugo, because that's normal in kids their age and Sadida are taller than most races, but making her look like an almost fully developed adult by the time she's in her early teens is too much.
Looking back, my personal gripe with her design not looking her age is that it really takes away from the impact of her post-time-skip look.
Yes, she's definitely grown into an even more beautiful and dignified princess, but other than her face and clothes there's not much difference because she doesn't really grow up that much.
I'll forever be mad my Sadida princess, whose family are all fucking giants, didn't come to outgrow most members of the Brotherhood of the Tofu. No, seriously.
I've had friends point out girls tend to stop growing in their early teens, meaning Amalia was never going to grow much taller than she was at 13. Which, fair. Except for the fact that Aurora is a random-ass Osamodas who's implied to be older than Amalia, and yet she still hit a growth spurt that made her a full head taller than her.
And for my final complaint...
Oh, Eva and Amalia's bond completely disappeared in the later seasons.
And the worst part is that because Eva's entire world came to revolve around Pinpin and their kids, the few times they interact or have any cute moments are all initiated by Amalia. Like her inviting her to stay over at the Sadida Kingdom in episode 2, or jumping to hug her when they were reunited in episode 11.
Between that and how little Eva seemed to care about Amalia's state and her father's health, it all reads as Amalia caring far more about their friendship than Eva.
It's as you said, if really does feel like, rather than growing closer, they grew apart.
