[OS X TeX] Typesetting equation w/o fractions appearing in smaller fontsize

[Michael Kubovy]

Dear Friends,

This:
\listfiles
\documentclass[]{minimal}
\usepackage{amsmath}

\begin{document}

\[
\mathcal{B}_{s}(x) =
\begin{cases}
\frac{r \log_{2} \frac{r}{(r + 1)} } {\log_{2} \frac{1}{(r + 1)} } &  
\text{if $x_{i}^{(2)} = y_{i}^{(2)}$;}\\
\frac{\log_{2} \frac{1}{(r + 1)} } {r \log_{2} \frac{r}{(r + 1)} } &  
\text{if $x_{i}^{(2)} \neq y_{i}^{(2)}$.}
\end{cases}
\]

\end{document}


appears as

How do I make all font sizes the same?

Here are the files used
  minimal.cls    2001/05/25 Standard LaTeX minimal class
  amsmath.sty    2000/07/18 v2.13 AMS math features
  amstext.sty    2000/06/29 v2.01
   amsgen.sty    1999/11/30 v2.0
   amsbsy.sty    1999/11/29 v1.2d
   amsopn.sty    1999/12/14 v2.01 operator names



_____________________________
Professor Michael Kubovy
University of Virginia
Department of Psychology
USPS:     P.O.Box 400400    Charlottesville, VA 22904-4400
Parcels:    Room 102        Gilmer Hall
        McCormick Road    Charlottesville, VA 22903
Office:    B011    +1-434-982-4729
Lab:        B019    +1-434-982-4751
Fax:        +1-434-982-4766
WWW:    http://www.people.virginia.edu/~mk9y/


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