Rami Arieli: "The Laser Adventure" Chapter 7 Section 2, page 8
The parameters which determines the Beam Divergence:

Figure 7.7a shows The parameters which determines the Beam Divergence:

W0 = Radius of the beam waist at the point z = 0.

W(z) = Radius of the beam at point z.
Figure 7.7a: Gaussian Beam Divergence.

Figure 7.7b explains graphically a comparison between a beam with narrow waist, and thus high divergence, to the same beam after expanding the beam waist, thus reducing the beam divergence (see chapter 8).

Figure 7.7b: High and Low Gaussian Beam Divergence.