They hold a lot more than what you may think.
Think of it like this, volume grows way faster than length, if you double the length of a cube, it can hold 8 of the original cubes (you can check this by stacking dice), and if you cuadruple the original length it can now hold 64 small cubes.
A truck can hold much more than you would imagine if you only thing of the length or are change.
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