Matrix doesn't shrink when put in fraction.

Question

This is my minimal example, the problem is the different size of the two x. I would expect the matrix to be shrunken too, but it isn't. Why is this and how is it corrected?

\documentclass[a4paper,12pt]{article}
\begin{document}
$\frac{\begin{pmatrix}x\end{pmatrix}x}{2}$
\end{document}

edit: I just realized: this is not matrix specific. \underbrace does it too. seems like the information to be smaller doesn't get through these "frames":

\documentclass[a4paper,12pt]{article}
\begin{document}
$\frac{\begin{pmatrix}x\end{pmatrix}x}{2}e^{\begin{pmatrix}x\end{pmatrix}x}\int_{\begin{pmatrix}x\end{pmatrix}x}$
$\frac{\underbrace{x}x}{2}e^{\underbrace{x}x}\int_{\underbrace{x}x}$
\end{document}

Answer 1

In this case you can use the smallmatrix environment of the amsmath package. Both x would have the same size. smallmatrix doesn't include delimiters, they have to be added.

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath}
\begin{document}
$\frac{\left(\begin{smallmatrix}x\\y\end{smallmatrix}\right)x}{2}$
\end{document}

Answer 2

Without having any solid knowledge on the issue, I trust the amsmath is doing The Right Thing. But I wanted to share some explorations on size and spacing. Personally, I'd fancy the bigl/r one.

$$
  {(x)x\over2} \qquad
  {\left(x\right)x\over2} \qquad
  {{x\choose y}x\over2} \qquad
  {\left({x\atop y}\right)x\over2} \qquad
  {\bigl({x\atop y}\bigr)x\over2} \qquad
  {\pmatrix{x\cr y}x\over2} \qquad
$$