Binomial coefficient parentheses are inconsistent across engines

Using the following code (run pdfLaTeX/XeLaTeX/LuaLaTeX), pdfTeX and XeTeX produce identical binomial coefficients, but LuaTeX makes larger parentheses in scriptstyle and scriptscriptstyle. Are these discrepancies by design, or are these engine-related bugs? How can I obtain unified results?

documentclass{article}
begin{document}
baselineskip=18pt

ifxdirectluaundefined
  ifxXeTeXcharclassundefined
    pdf/TeX%
  else
    XeTeX%
  fi
else
    LuaTeX%
fi

$displaystyle      {nchoose k},
 textstyle         {nchoose k},
 scriptstyle       {nchoose k},
 scriptscriptstyle {nchoose k}$
end{document}