ScriptSizeMultipliers is an option for StyleBox which specifies how much smaller to render each successive level of subscripts, etc.
ScriptSizeMultipliers is applied to FontSize for characters that appear in constructs such as subscripts, superscripts, underscripts, overscripts and built-up fractions.
The default setting for ScriptSizeMultipliers is 0.71, yielding approximately a factor 2 reduction in character area at each level.
ScriptSizeMultipliers -> , , ... , uses multiplier for level i, and multiplier for levels n and beyond.
See The Mathematica Book: Section 2.8.11.
See also: ScriptMinSize, ScriptBaselineShifts.