For me tarps have evolved along two fronts: Evolution of tarps follows the evolution of hammocks. When I started buying my first hammocks they were in general 10' long. Over time hammocks got longer, I need bigger tarps to cover them.
As I have grown in this recreation of ours i tend to want to hang in different seasons than I did when I started. In the beginning a smaller tarp was fine as i was not really pushing myself or the weather. Now that i like to hang in winter I use some very large tarps just to give me coverage from biting winds and blowing snow. So I do think the evolution of tarps keeps pace with what we do out in the wild as well.