Based on what I have observed, it's all about color. For example, I convinced my father to grow a beard this summer and at first it didn't look extremely thick. However, this visual effect was simply due to the fact that his beard is pretty much all gray now. Once it got some length it suddenly became a thick, hairy monster, and he refuses to part ways with it despite my mom's hatred of facial hair. When my dad was younger and his hair was still dark, although he didn't grow out his beard, his stubble appeared thicker when actually it was just because the hairs were darker and more easily visible at shorter lengths.

The only other conclusion I can draw from this observation is that hopefully one day my beard will be half as thick as my dad's...