\useMPlibrary[dum]

\starttext

    \placefigure
      {Some very small figure with not too much text.}
      {\externalfigure[dummy][width=5cm,height=2cm]}

    \input tufte \page

    One with a lot of text. In order to distinguish the caption from
    the runing text:

    \placefigure
      {\input tufte\relax}
      {\externalfigure[dummy][width=\textwidth,height=.2\textwidth]}

    \input tufte \page

    So this is nicer:

    \setupcaptions
      [maxwidth=.75\hsize]

    \placefigure
      {\input tufte\relax}
      {\externalfigure[dummy][width=\textwidth,height=.2\textwidth]}

    \input tufte \page

\stoptext

