begin{document}
selectlanguage{english}
tableofcontents{}

section{small{Introduction}}

section{small{Chapter 1}}

documentclass[10pt,a5paper]{extarticle}


usepackage[english]{babel}
usepackage{tocloft}
usepackage{blindtext}

makeatletter
renewcommandsection{@startsection {section}{1}{z@}%
                                   {-2.5ex @plus -1ex @minus -.2ex}%
                                   {0.3ex @plus.2ex}%
                                   {normalfontsmallbfseries}}

renewcommandsubsection{@startsection{subsection}{2}{z@}%
                                     {-3.25ex@plus -1ex @minus -.2ex}%
                                     {1.5ex @plus .2ex}%
                                     {normalfonttinybfseries}}
makeatother




renewcommand{cftsecfont}{small}
renewcommand{cftsubsecfont}{tiny}

begin{document}
tableofcontents
clearpage
section{Foo}
blindtext
section{Foobar}
blindtext
end{document}