Are the Storm Giant's strength scores in the Monster Manual wrong?

Question

The Storm Giant has a STR score of 29. According to the encumbrance rules that means the maximum weight a giant could lift is 435 lb. which really isn't that much.

To put it in perspective. The Storm giant is 5x taller than a human, at the same proportion and thus has about 25x more mass. An average human male weighs around 180lb. A Storm Giant thus weighs around 4500 lb. So it's impossible for one Storm Giant to carry another.

Another way of looking at it. A human-scale sword might be around 4ft. long and weigh 3 pounds. However, a sword that's long enough for a Storm Giant would need to be around 20 ft. long and to maintain its own integrity would need to be about 2 inches thick at its centre and might be a foot wide. A sword like that would weight about 800 lb.

To even be able to wield a sword like that without being encumbered, the Storm Giant would need a minimum STR score of 160.

So in conclusion, Storm Giants must be about the weakest creatures in the Multi-verse on a pound for pound basis :P

Am I missing some encumbrance rule somewhere that adjusts this based on creature size category?

Answer 1

Storm Giants are Huge creatures, not Medium.

The encumbrance rules neglect the size of a creature when calculating if it is encumbered or heavily encumbered. While this is true, the maximum capacity of a Storm Giant is not the same that a human would have given it a Strength score of 29.

Quoting the rules on lifting and carrying (Player's Handbook, page 176):

Carrying Capacity. Your carrying capacity is your Strength score multiplied by 15. This is the weight (in pounds) that you can carry, [...].

[...]

Size and Strength. Larger creatures can bear more weight, whereas Tiny creatures can carry less. For each size category above Medium, double the creature's carrying capacity and the amount it can push, drag, or lift. [...]

The rules on creature size categories are on page 191 of the Player's Handbook:

\$\begin{array}{|l|c|} \hline \textbf{Size} & \textbf{Space} \\ \hline \text{Tiny} & 2\,\frac{1}{2}\,\text{by}\,2\,\frac{1}{2}\,\text{ft.} \\ \text{Small} & 5\,\text{by}\,5\,\text{ft.} \\ \text{Medium} & 5\,\text{by}\,5\,\text{ft.} \\ \text{Large} & 10\,\text{by}\,10\,\text{ft.} \\ \text{Huge} & 15\,\text{by}\,15\,\text{ft.} \\ \text{Gargantuan} & 20\,\text{by}\,20\,\text{ft. or larger} \\ \hline \end{array} \$

Huge size is two size categories above Medium size; therefore, the maximum weight a storm giant could lift is not 435 lbs., but four times that (i.e. 435 lbs., doubled twice): a total of 1,740 lbs.

---

Remember that the Encumbrance rules are a variant rule and are meant to apply to playable races (Small- to Medium-sized creatures). The DM is free to adjust those values to be doubled for each size above Medium to keep it plausible to be used for larger creatures, to mirror the increase in carrying capacity for larger sizes.

Also, they are monsters. The rules for players do not necessarily apply the same way for NPCs.

Answer 2

So in conclusion, Storm Giants must be about the weakest creatures in the Multi-verse on a pound for pound basis :P

If you apply real-world physics of scaling as you attempted, this claim is generally right. That has to do with the square-cube law, which is summarized in the Science World article "What If Humans Were Giants?" as follows:

The square-cube law states that as an object grows or shrinks in size, its volume grows and shrinks faster than its surface area—respectively. This law applies to any animal, since an animal that is twice as tall, compared to a similar animal, will experience the cross sectional area of its leg bones become four times as large, but have its weight become eight times as large. One’s muscles, bones and organs would not strengthen concurrently and as a result, their legs wouldn’t be able to support the weight of the rest of their body.

In order for your bones to support the additional weight, the cross-section area would have to become 8 times as much instead of 4 times, and such a create would have quite thick thighs.

In pictures, storm giants have similar proportions of leg to body size as humans, and that means they would be quite weak and fragile in the real world. They would probably not be able to support their own weight. There are some tricks nature uses to get around this, like lighter bones and stuff, but not to this extent.

Even with Kogarashi Kaito's answer that the carrying capacity would be 1,740 lbs. (not 435 lbs.), that value is still comparably small if you calculate a storm giant's weight.

So in conclusion, do not apply real world physics to D&D.

I'm not an expert with this; my knowledge comes from YouTube videos. If you want to know the exact information on the square-cube law, please do your own research. I know the gist is right and this creature wouldn't be able to stand.