\big delimiters not working with scrartcl?

Question

I noticed that sometimes the \big delimiters stop working. One example is when scrartcl is the document class.

\documentclass{scrartcl}

\begin{document}
\begin{equation}
    (\frac{1}{2})
    \bigl(\frac{1}{2}\bigr)
\end{equation}
\end{document}

I obtain the following result (i.e., the \bigl/\bigr macros have no effect):

(The bigger delimiter macros such as \Big and \bigg seem to work though.)

Another example is when loading both breqn and mathtools:

\documentclass{article}

\usepackage{mathtools}
\usepackage{breqn}

\begin{document}
\begin{equation}
    (\frac{1}{2})
    \bigl(\frac{1}{2}\bigr)
\end{equation}
\end{document}

Is this a bug? What would be a possible workaround?

Answer 1

The way \big is defined in the LaTeX kernel does not make it scale when the main font size is increased: here's the code from fontmath.ltx

524 \def\big#1{{\hbox{$\left#1\vbox to8.5\p@{}\right.\n@space$}}}
525 \def\Big#1{{\hbox{$\left#1\vbox to11.5\p@{}\right.\n@space$}}}
526 \def\bigg#1{{\hbox{$\left#1\vbox to14.5\p@{}\right.\n@space$}}}
527 \def\Bigg#1{{\hbox{$\left#1\vbox to17.5\p@{}\right.\n@space$}}}

Then \bigl( is just \mathopen{\big{(}}. You can see that the size of the delimiters does not depend from the main font size, but it's hardwired. Thus, when 11pt or 12pt size is used, \big ends up not being distinguishable from the normal size of the delimiter.

The amsmath package obviates to this by defining

552 \renewcommand{\big}{\bBigg@\@ne}
553 \renewcommand{\Big}{\bBigg@{1.5}}
554 \renewcommand{\bigg}{\bBigg@\tw@}
555 \renewcommand{\Bigg}{\bBigg@{2.5}}
556 \def\bBigg@#1#2{%
557   {\@mathmeasure\z@{\nulldelimiterspace\z@}%
558      {\left#2\vcenter to#1\big@size{}\right.}%
559    \box\z@}}
560 \addto@hook\every@math@size{%
561   \global\big@size 1.2\ht\Mathstrutbox@
562   \global\advance\big@size 1.2\dp\Mathstrutbox@ }
563 \newdimen\big@size

and defining suitably \Mathstrutbox@. Essentially the same is done by the exscale package. Nothing is done to this by mathtools, so the relevant package is amsmath.

I believe there is a bug in the breqn package, because \big@size is not set to the same value as with amsmath: with this example input

\documentclass{article}

\usepackage{amsmath}
%\usepackage{breqn}

\begin{document}
\begin{equation}
\makeatletter\showthe\big@size\makeatother
\end{equation}
\end{document}

I get

> 11.99995pt.
l.8 \makeatletter\showthe\big@size
                                  \makeatother

while uncommenting the call to breqn gives

> 7.20004pt.
l.8 \makeatletter\showthe\big@size
                                  \makeatother

and this difference explains why \bigl( is not larger than the simple (.