The terms SD, MSD, and Yo-SD were coined by the originator of the modern species of ball joint dolls, Volks. They are acronyms that stand for Super Dollfie, Mini Super Dollfie, and "Yo-is dat a tiny doll er what?" While in may circles these terms have become synonymous with their various size scales (like a xerox, or kleenex), Volks rejects this notion and many manufacturers have their own names for these size classes. If you speak about SDs, MSDs, or Tinies (Volks Yo-SDs are mostly small children and "Tinies" covers all the dolls of that size, including many of the animal varieties), all but the snobbiest of collectors will acknowledge what you're talking about.
The term dollfie is a contraction of the words "doll" and "figure", which makes sense when you find out that Volks got into the ball joint doll racket by branching out from their line of cast resin figures, which are made of similar materials. The original Dollfies were 1/6 scale then logically, if you were to make a larger 1/3 scale Dollfie it would be "Super". It is my understanding that according to Volks, their specific size categories represent the doll's age and there are more sub sizes within the various scales (SD13, SD16, etc.).
For a more in depth look at Volks and their dolls, go here:
http://www.angelden.net/volks/vocab.php