Strength is proportionate to the square of the linear dimension, but mass is proportionate to the cube of the linear dimension. The surface area of a cube of size x is 6x2, while the volume is x3. Thus, doubling linear dimension will increase surface area by a factor of four and volume by a factor of eight.
This is why making an object larger makes it stronger, but it makes it heavier than it makes it strong. At some point, you reach the point of diminishing returns at which the force needed to support the mass of the object exceeds its tensile strength and the structure collapses under its own weight. This is why the giants of folklore could not have been fifty feet tall, and by the same token, why insects and arachnids generally can only be very small; An exoskeleton can only support so much weight, and it was necessary for life forms to evolve an internal skeleton to grow larger because it distributes weight.
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