\setupbodyfont[dejavu]

\setupbodyfontenvironment
  [default]
  [smallsized=1.25,
   mediumsized=1.5,
   largesized=2.0]

\definefontsize[smallsized]
\definefontsize[mediumsized]
\definefontsize[largesized]

\starttext

\setuphead[chapter]   [style=\bflargesized]
\setuphead[section]   [style=\bfmediumsized]
\setuphead[subsection][style=\bfsmallsized]

\chapter   {Whatever $x-1$}
\section   {Whatever $x-1$}
\subsection{Whatever $x-1$}

\input ward

\stoptext

