As a last resort, you can use \pdfliteral (at least in pdflatex and lualatex, IIRC), altough it may not look as good in printing. Just modify the 0.25 bit to your taste.
\documentclass{scrartcl}
\newcommand*\mathbold[1]{\pdfliteral direct{2 Tr 0.25 w}#1\pdfliteral direct{0 Tr 0 w}}
\begin{document}
\[
\sum_{n=1}^{\infty} \frac1{n^2} = \frac{\pi^2}6. \qquad \mathbold{\sum_{n=1}^{\infty} \frac1{n^2} = \frac{\pi^2}6.}
\]
\end{document}

By the way, I think that what you are showing might be an artifact of the svg rendering.
\boldmathbefore entering math mode. – Bernard Jul 10 '16 at 15:23\boldmathwas first reaction too. However,\boldmathinvokes bold-extended by default, whereas the wikipedia example appears to use "regular-width" bold math fonts. – Mico Jul 10 '16 at 15:29\pdfliteralartifact. – Manuel Jul 10 '16 at 15:40