Assuming the two crescent wrenches have the same shape and material, the weight (mass) of an object is proportional to its volume, which is proportional to the cube of its dimensions. Therefore, the 16 inch crescent wrench will be 2 times longer than the 8 inch crescent wrench, and its weight will be 2^3 = 8 times heavier than the 8 inch crescent wrench.
So, the 16 inch crescent wrench is 8 times heavier than the 8 inch crescent wrench.