Complex sentence: Here, the size of the scanning volume is determined by the spot size (close to diffraction limit) of the optical system because the image of the scanning laser is not an infinitely small point but a three-dimensional diffraction pattern.
Split into simpler sentences: Here, the size of the scanning volume is determined by the spot size (close to diffraction limit) of the optical system. This is due to the fact that the image of the scanning laser is not an infinitely small point but a three-dimensional diffraction pattern.