The Collaborative International Dictionary of English v.0.48:

Sponginess \Spon"gi*ness\ (sp[u^]n"j[i^]*n[e^]s), n.
   The quality or state of being spongy. --Dr. H. More.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sponginess
    n 1: the porosity of a sponge
    2: the property of being able to occupy less space [syn:
       compressibility, squeezability, sponginess] [ant:
       incompressibility]