The Collaborative International Dictionary of English v.0.48:

Diffusiveness \Dif*fu"sive*ness\, n.
   The quality or state of being diffusive or diffuse;
   extensiveness; expansion; dispersion. Especially of style:
   Diffuseness; want of conciseness; prolixity.
   [1913 Webster]

         The fault that I find with a modern legend, it its
         diffusiveness.                           --Addison.
   [1913 Webster]