thinmuskip does not appear to be getting smaller in scriptstyle.
the plain Tex file
\tracingonline1
\setbox0\hbox{$\scriptstyle\log x$}\showbox0
\bye
produces
> \box0=
\hbox(4.8611+1.3611)x16.24425
.\mathon
.\hbox(4.8611+1.3611)x10.34378
..\sevenrm l
..\sevenrm o
..\sevenrm g
..\kern0.10762
.\glue(\thinmuskip) 1.36574
.\seveni x
.\mathoff
in pdftex but in luatex 0.89.4 (today's build from texlive svn)
> \box0=
\hbox(4.8611+1.3611)x16.54514, direction TLT
.\mathon
.\hbox(4.8611+1.3611)x10.34378, direction TLT
..\sevenrm l
..\sevenrm o
..\sevenrm g
..\kern0.10762
.\glue(\thinmuskip) 1.66663
.\seveni x
.\mathoff
1.66663 is the value of \thinmuskip at \textstyle (cmr10)
David
