dnd 5e – Are the Storm Giant’s strength scores in the Monster Manual wrong?

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.