Take the new size of the object or weapon as a fraction of it’s old size and cube it. That is the new weight. For example, an object is increased by 50%. It’s new size is 1.5 times it’s original size. 1.5 cubed is 3.375 so it ways 3.375 times it’s original weight.
I believe a table is listed under alter self or enlarge or one of those size-altering spells.
Roger Bacon