Avec les classes standards, standard, tout comme les autres subdivisions, vous pouvez modifier
la commande `\l@chapter`.
\documentclass[french]{book}
\usepackage{lmodern}
\usepackage[T1]{fontenc}
\usepackage{babel}
\makeatletter
\renewcommand*\l@chapter[2]{%
\ifnum \c@tocdepth >\m@ne
\addpenalty{-\@highpenalty}%
\vskip 1.0em \@plus\p@
\setlength\@tempdima{1.8em}% % ---<--1.5em
\begingroup
\parindent \z@ \rightskip \@pnumwidth
\parfillskip -\@pnumwidth
\leavevmode \bfseries
\advance\leftskip\@tempdima
\hskip -\leftskip
#1\nobreak\hfil \nobreak\hb@xt@\@pnumwidth{\hss #2}\par
\penalty\@highpenalty
\endgroup
\fi}
\renewcommand*\l@section{\@dottedtocline{1}{1.8em}{2.8em}}%
\renewcommand*\l@subsection{\@dottedtocline{2}{4.6em}{3.2em}}%
\renewcommand*\l@subsubsection{\@dottedtocline{3}{7.0em}{4.7em}}%
\renewcommand*\l@paragraph{\@dottedtocline{4}{10em}{5.5em}}%
\renewcommand*\l@subparagraph{\@dottedtocline{5}{12em}{6.5em}}%
\makeatother
\begin{document}
\tableofcontents
\chapter{foo bar baz}
\section{Bla bla}
\chapter{foo bar baz}
\chapter{foo bar baz}
\chapter{foo bar baz}
\chapter{foo bar baz}
\chapter{foo bar baz}
\chapter{foo bar baz}
\chapter{foo bar baz}
\chapter{foo bar baz}
\chapter{foo bar baz}
\chapter{foo bar baz}
\section{Bla bla}
\chapter{foo bar baz}
\end{document}
