The Hulk’s size is constantly changing in the MCU because different Directors have different ideas for what he should look like.
That being said, his size hasn’t actually changed all that much.
In The Incredible Hulk we see a more “body builder-esque” Hulk. This is what the Hulk tends to look like in the comics. It’s the typical male fantasy for what an extremely strong man looks like.
This is what I’ve come to learn body builders refer to as their “dry look”. This is actually what a human being looks like when they’re near their weakest. They’re typically very dehydrated so that they aren’t carrying any extra water weight, and their muscles will be as defined as possible. They’re at their lowest body fat percentage.
While it certainly makes Hulk look like a monster, it’s the least likely to be what he would realistically look like if he had any human physiology at all.
The next time we see him is in Marvel’s The Avengers. In this, he looks a bit chunkier. Overall he’s a little taller, and more heavily built. This look continues into Age of Ultron.
As you can see, he’s got some fat on his body, and his muscles don’t have the “corded” look they had in TIH. However, this is far more realistic for what a “strongman” looks like. When you have a lot of muscle, you need a layer of fat to use as energy to fuel your body. If you take a look at the world’s strongest man, you’ll see he definitely doesn’t look like a body builder.
This is Eddie Hall. He won title of World’s Strongest Man in 2017. This is Hafthor Bjornsson, he won in 2018.
And the current year’s WSM, Martin Licis.
As you can see, none of them look much like the stereotypical “strong superhero” but they’re all far stronger than any of you reading this. (Well, probably most of you. Maybe 1 or 2 of you are near their strength.)
Then we come to Thor Ragnarok. Hulk is looking a bit leaner, but still a big bulky guy.
He’s got a little more definition but looks relatively the same as he has since Marvel’s The Avengers.
The Hulk does not change size with anger. This was an effect they chose to use in the 2003, Ang Lee movie Hulk to help the viewer understand where Hulk was at with his strength gains. It is not comic book accurate.
While some people believe it should be, that doesn’t change the fact that it is not.
The Hulk changes size throughout the MCU movies because the directors/editors have differing ideas of what Hulk should look like in their movie, and that ultimately decides how he looks.
(I chose not to include Merged Hulk because his size change is due to the merger between Hulk and Banner, and isn’t very noticeable anyway since he’s almost always wearing clothes.)
For those of you who might tell me that this question is referring to Hulk’s height, I will answer you here. His height changes because the directors have decided his height should change. That being said, I don’t think it has. In the Avengers, he slouches forward, creating the illusion that he’s shorter than he is. In Ragnarok, he stands up straight. This shows his true height. In Endgame, he might be a little bit smaller. That’s because the Russos hate Hulk. Ignore it.
Source : Quora
