Crop a float containing subfigures horizontally with Preview or Standalone package

Question

I have a float containing subfigures. I want to "export" it as a single item so that I can include it elsewhere as one object. I tried to use the Preview package with [active,floats,tightpage], but it only makes the page tight vertically - it leaves significant white space to the right of the second subfigure in the example below:

\documentclass{article}
\usepackage{mwe}

\usepackage{subcaption}

\usepackage[active,floats,tightpage]{preview} % include this last

\begin{document}

\def\FigSize{.33}
\begin{figure}
  \begin{subfigure}[b]{\FigSize \textwidth}
    \includegraphics[width=\FigSize \textwidth]{example-image-a}
  \end{subfigure}
  \begin{subfigure}[b]{\FigSize \textwidth}
    \includegraphics[width=\FigSize \textwidth]{example-image-b}
  \end{subfigure}
\end{figure}

\end{document}

Is there a way to make it crop in the horizontal direction as well?

I have also tried this:

\documentclass[preview]{standalone}
\usepackage{mwe}
\usepackage[]{subfig}

\begin{document}

\def\FigSize{.23}
\begin{figure}[htbp!]
\centering
\subfloat[]
  {
  \includegraphics[width=3cm]{example-image-a}
  }
\subfloat[]
  {
  \includegraphics[width=3cm]{example-image-b}
  }
\end{figure}

\end{document}

but the result is similar in that the output is still the full page width rather than being tight to the two subfigures.

------------- Edit ----------------

As Peter Grill suggested, adding 'varwidth' does indeed fix the simple demo. In my real case, I am using subfloats from subfig:

\documentclass[preview,varwidth]{standalone}
\usepackage{mwe}
\usepackage[]{subfig}

\begin{document}

\def\FigSize{.23}
\begin{figure}[htbp!]
\centering
\subfloat[]
  {
  \includegraphics[width=3cm]{example-image-a}
  }
\subfloat[]
  {
  \includegraphics[width=3cm]{example-image-b}
  }
\end{figure}

\end{document}

The cropping is still correct, but the (a) and (b) below the figures are not centered on the figures as they should be?

Answer 1

As pointed out by Peter Grill, using varwidth as an option to standalone helps fitting images with boundary as per the width. But for reasons beyond my understanding, the captions are aligned to the left. As a cure, one can load caption package with the option justification=centering.

Code:

\documentclass[varwidth]{standalone}
\usepackage{mwe}
\usepackage[labelfont=normalsize,labelformat=parens,
justification=centering]{caption,subfig} %%% Center the caption
\begin{document}

%\def\FigSize{.23}
\begin{figure}[htb!]
\subfloat[]{%
\includegraphics[width=3cm]{example-image-a}
}%
\hspace{3pt}%
\subfloat[]{%
\includegraphics[width=3cm]{example-image-b}%
}%
\end{figure}
\end{document}

Or

\documentclass[varwidth]{standalone}
\usepackage{mwe}
\usepackage[labelfont=normalsize,labelformat=parens,
justification=centering]{caption,subfig} %%% Center the caption
\begin{document}
\begin{minipage}{6.2cm}
\begin{minipage}{3cm}
\includegraphics[width=3cm]{example-image-a}
\captionof{subfigure}{}
\end{minipage}%
\hfill
\begin{minipage}{3cm}
\includegraphics[width=3cm]{example-image-b}%
\captionof{subfigure}{}
\end{minipage}%
\end{minipage}
\end{document}