The skin of a mature goat holds 30 to 40 liters.
A liter of wine weighs 1 kilogram, about 2.2 lbs.12.2046226218488 lbs.
So Elijah and Nathan lifted wineskins which weighed 30-40 kilos, or 66-88 lbs.
The normal donkey load of two full goatskins weighed 60-80 kilos, or 132-198 lb.
This modern rack and modern wineskin both look much smoother than those I imagine Elijah and Nathan saw.