A 21' foot humanoid is totally huge as it is. Eighteen foot tall Cloud Giants are just huge. A 110lb sword, 14' foot long, and 8" broad is huge. Nothing bigger is needed to terrify, and in fact any bigger would cease to. For me, bumping them up 24' tall shrinks them. They move from inhabiting a fantasy world to a cartoon world where scale doesn't matter and is likely to change from scene to scene anyway.
Huge size is actually an important break point in game play terms for enemy creatures since it is two size categories larger than medium and there are no large sized PC races.Stop right there and tell me in 5e exactly what game mechanics are strongly effected by the extra 5 feet of height given to the giants. How does it actually effect play?
12’ is 2x the height of The average human. And 4 feet taller, or 50% larger, then the tallest human ever. That is not insignificant.1e/3e giants weren’t giant.
A 12’ tall human-shaped person is a few feet taller than the tallest human in living memory. Nowhere near the difference of scale that these creatures should represent. The greatest giants should have faces bigger than the tallest natural human.
When I think of the iconic storm giant, he is rising from the deep, a whole ship comparable in scale to his face!
The smaller true giants should be such that they can interact in fairly normal ways with a human. Half-giants should seem possible, but not with the greatest giants.
I’ve a cloud giant mini that seems to be wielding architecture as a sort of mace. She isn’t quite big enough compared to the human minis, but the visual is still evocative and I don’t expect minis to have the right proportions anymore (ancient dragons are tiny things).
Why even have these creatures if they aren’t something that must be dealt with differently than something like an ogre?
Huge size is actually an important break point in game play terms for enemy creatures since it is two size categories larger than medium and there are no large sized PC races.
You can move through the space of any creature two size categories smaller than you. Giants get the ability to move through the space of any PC which can be pretty huge for area control. In addition to that you cannot grapple or shove any creature that is more than one size category than yourself so by default you cannot grapple or shove a giant. Your fighter and barbarian want to hold the line to protect the squishy wizard? Well they're going to have a difficult time of that since giants can just step right over them. Which for giants is very fitting. If you have a creature whose very definition is that they are "like people but big", they should be of a size where that matters.
For me it comes down to not believing that people who want bigger giants are capable of imagining just how big a 12' tall humanoid actually is.
12’ is 2x the height of The average human. And 4 feet taller, or 50% larger, then the tallest human ever. That is not insignificant.