Say you have a filament that's 1 µm in diameter, and 1 meter long. You want to fill up a 1m^3 (1m W x 1m H x 1m L) space with these, how many of these can you place in such a space? Over a trillion! And thus, the combined km length of these will also be over a billion km. At such small scales things can become very long when summed up.