Mike looks up to Will. Literally, and most of the time?? I looked through seasons 2-4 and this happens during most of their scenes together. Mike, who's several inches taller than Will, somehow always arranges himself to look at Will from a lower angle, or at almost equal height.
The boy has bad posture in an effort to... what? To be able to see Will's face better? To make sure Will knows that Mike doesn't figuratively look down on him, doesn't baby him? To make sure Will knows they're equals? To close the distance between them? To not seem bigger or taller than Will? To make sure Will knows he's seen and understood? All of the above?
Mike leans down to look Will in the face:
And he sits down so he can be on Will's level or look at Will from below:
Notably for the above pictures from the hospital in s2, we see that Will and Mike look fairly level in the second picture, but in the first, it's shot so that Mike is looking from such a lower angle. So this seems to be intentional.
Here are some of Mike and El to compare. Mike has much straighter posture in scenes with El, leaving him taller, further away from El, resulting in him not looking El in the face as often. We see this exacerbated in s4, as Mike is even taller, the distance is further, and he also isn't looking at El much with good eye contact or without sunglasses or the sensory deprivation glasses. When seeing Will and Mike at the same height, it feels like an equal relationship, but seeing Mike towering over El makes their relationship feel less equal. He's not looking her in the face as often in an effort to understand her.
We see a lot of Mike and Lucas together in s3, and Mike also stands fairly tall compared to Lucas in their shots together, making any height difference obvious. In the picture set below, we see that, but also some side by sides of how Mike stands when talking with Lucas, and then how Mike talks with Will just seconds later. You can see how different it is: Mike adjusts for the height difference with Will in a way we don't see him do for anyone else.
So I don't think it's just that Finn Wolfhard is too tall and he's made to lean over to make it easier to film multiple people of varying heights. It's a Mike and Will thing.
Another couple comparisons:
The rain fight: Mike stands taller in their rain fight at the beginning when he's on the defensive, but he gets lower and closer to Will the further into the fight they get, when they're saying the more important things to each other.
The phone booth: We can also see Mike bending over to look at Will in the phone booth, but standing straight and further away when Jonathan is listening to the phone.
The movie theater: when they're first sitting down in the theater, maybe when people are still looking at them, Mike sits tall, but he sits lower once the movie has started and he wants to talk with Will.
On that note, Mike stands taller around Will when in public or around others he doesn't feel as comfortable around. This might be to hide aspects of his relationship with Will, or to be protective towards Will. I didn't include these shots, but Mike also stands tall when protecting Will from Dart in s2, when he's with Will in the field after Will is possesed in s2, and in the gunfight in s4.
But one on one with Will, talking with Will, listening to Will, having heart to hearts with Will, he scrunches his back so much so that he can be at equal height or lower than Will to look Will in the face or to look up to him.
They hired Noah and Finn knowing that Noah was younger and shorter, so I don't think they meant for Will to be a taller character.
This is just their dynamic. Mike is the tall one but he wants Will to be able to feel like the tall one.
