If the answer is only inside the texinfo.tex file, anybody knows more
exactly where?
You guessed it. Inside texinfo.tex, around line 2052, there's this
set of definitions:
% Chapter (and unnumbered) fonts (17.28pt).
\def\chapnominalsize{17pt}
\setfont\chaprm\rmbshape{12}{\magstep2}{OT1}
\setfont\chapit\itbshape{10}{\magstep3}{OT1IT}
...
\font\chapsy=cmsy10 scaled \magstep3
\def\chapecsize{1728}
I suppose you can increase all the \magstep values to whatever you like.
(This is assuming the default 11pt text size. There's a second sequence
of definitions for the 10pt case, a few hundred lines below.)
Hope this helps,
karl
