Not a short s, but a z, as the name indicates:  es-zett, S-Z.  This reflects the use of z in Old and Middle High German to represent a sibilant sound distinct from s, derived from /t/ by the High German sound shift but distinct from original /s/.  When the distinction was lost in the 13C, z came to be used for its modern sound /ts/, but the ligature came to represent the merged /s/.