Why doesn't math \fontdimen's work with XeTeX?

Question

Why doesn't the \fontdimen-command change anything with XeTeX?

\input pgffor
\def\chfndims#1{\foreach\dim in{13,...,19}{\fontdimen\dim#1=20pt}}
\font\mymathfn="XITS Math:script=math" at 10pt \chfndims\mymathfn
\font\mymathfns="XITS Math:script=math;+ssty=0" at 7pt \chfndims\mymathfns
\font\mymathfnss="XITS Math:script=math;+ssty=1" at 5pt \chfndims\mymathfnss
\textfont0=\mymathfn \scriptfont0=\mymathfns \scriptscriptfont0=\mymathfnss
\textfont1=\mymathfn \scriptfont1=\mymathfns \scriptscriptfont1=\mymathfnss
\textfont2=\mymathfn \scriptfont2=\mymathfns \scriptscriptfont2=\mymathfnss
\textfont3=\mymathfn \scriptfont3=\mymathfns \scriptscriptfont3=\mymathfnss
\textfont4=\mymathfn \scriptfont4=\mymathfns \scriptscriptfont4=\mymathfnss
\textfont5=\mymathfn \scriptfont5=\mymathfns \scriptscriptfont5=\mymathfnss
\textfont6=\mymathfn \scriptfont6=\mymathfns \scriptscriptfont6=\mymathfnss
$ a^b \, c_d \, e^1 \, f_2 $
\bye

Nothing changes.

I would like to change the horizontal space before a superscript.

Answer 1

There's no font dimension parameter that controls the spacing before a subscript/superscript: they are attached to the atom they refer, possibly shifted horizontally based on the information provided by the atom, if it's a character box. The parameter \scriptspace controls the space after them.