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?