DSB, I've never seen a small base sizer die described as you describe it. I think you are misunderstanding RCBS' description of what a small base sizer die is.
Q. I see a Small Base Die Set listed for my caliber. Do I need these or should I buy a Full Length Die Set or Neck Die Set? How does each set differ?
A. The Small Base Die set is intended for use for ammunition to be used in auto, semi-auto, and lever action rifles so that the loaded round chambers and extracts easily. The Small Base Sizer Die sizes the case from the shoulder to the head of the case a couple of thousandths smaller than a Full Length Sizer Die. In certain calibers it also sets the shoulder of the case back a thousandth or two more than the Full Length Sizer Die. The Full Length Die Set or Neck Die Set is not normally recommended for ammo to be used in auto, semi-auto, or lever action rifles. The Full Length Die set is recommended for ammunition used in bolt action rifles, particularly for ammunition to be used for hunting. The Neck Die Set can also be used to produce ammunition for use in bolt action rifles. The Neck Sizer Die sizes only the neck of the case so it will hold the bullet firmly. It does not size the body of the case nor does it set the shoulder back. Neck sized cases will usually chamber for three or more firings, depending on the powder charge and chamber dimensions. However, over a period of time, a slight drag will be noticed when the bolt is locked. At this point, cases will need to be full length sized and the shoulder set back so they will chamber and extract easily.
The above is from RCBS' FAQ section. A small base sizer die is a full length sizer die that specifically sizes the shoulder to the head a bit smaller than a normal Full Length sizer die but still sizes the neck. If the small base die worked as you understand it, there would be little or no neck tension to hold the bullet. It would not be a successful sizer die.

If you have a small base die set, a full length die sizer is doing nothing for you. Well, except giving you a sizing cycle that is a bit less "work" of the brass than the other cycles.