Animator here. Allow me to elaborate.
The biggest reason is presentation. Despite it being cartoons, animators still have to strive for it being realistic in the real world type of way. It still has to be believable, even if the show is about a bunch of super humans fighting against a ruthless villain who's going to blow up the planet.
Much like cartoon characters, real life babies have much more exaggerated proportions. Bigger eyes, bigger ears, bigger heads, smaller bodies, etc etc. The eyes and ears and head you're born with are the eyes and ears and head you're stuck with for life (they will not grow!) so that's why they're exaggerated so much on young cartoon characters (ex. Timmy Turner from Fairly Odd Parents). So no, it's not your eyes "adjusting their size", it's literally just the rest of you growing and so your eyes look smaller in comparison (same for your head and ears).
Same being for adult characters and real life adults. As you get older, the size of your body stretches out, and then what was once considered big eyes and big ears and a big head now look smaller by comparison because of the way the body has grown. It's easy to see this if you compare a baby to an adult.
Real life Child Vs. Adult:
Cartoon Child Vs. Adult:


When you're presenting a character in a new light, grown up, you have to make people understand with only pictures that they have grown up. If you keep those big baby eyes and ears and head on a body that has grown, the proportions are now off, and in that sense, the character hasn't even "grown up" - they've just somehow grown an extra 5-6 feet. I mean, looking at an adult character with the same big features they had as children would be the same as looking as an adult person in real life with the same big features they had as children, if they grew with their bodies. It would be disturbing and even weirder than your original point in the OP.
So that's why. There's also other reasons, like portraying innocence and hardships and all that, but most of it is the literal, stemming to keeping proportions and making it realistic and make sense, which I can't stress enough, is so so important, even if you're drawing weird alien creatures or cartoon boy scouts. I think because it's a cartoon show, most viewers just shove that stuff aside because they don't think it's important in the grand scheme of things, but it's so ridiculously important, I can't even cover all the importance of it in this one post.
You'll have a hard time finding an actual, real animator (not a freelancer who doesn't have experience and doesn't even know what they're getting themselves into) who can't draw realism and bodies to proportion, and that's because of this. In fact, pretty much all animators are required to have at least one full year of life drawing classes under their belt, and all the famous and master animators (cough Glen Keane, Richard Williams cough) have even more (not just 2D either; 3D as well.) And this goes far beyond just animation - it applies to comics as well.
Yeeep. That's pretty much it.