Math formula changes in tikz environment

I am trying to write a formula in a tikzpicture but it gets changed. The indices for the cumulative product is moved from under the \prod to after. See MWE:

\documentclass{beamer}
\usepackage{tikz}

%%%%%%%%%%%%%%%%%
\begin{document}

\begin{frame}

\begin{center}
\begin{tikzpicture}
\node [font=\bf\tiny] {$1 - \widehat{S}(t) = \prod_{j:t_j \leq t} \frac{d_{kj}}{n_{j}}$};
\end{tikzpicture}
\end{center}

$$1 - \widehat{S}(t) = \prod_{j:t_j \leq t} \frac{d_{kj}}{n_{j}}$$

\end{frame}

\end{document}


Any idea why? Any way around?

-
The formula in the tikzpicture environment is typeset in text style; so \prod\limits_{...} is the answer. Or $\displaystyle<formula>$ if you want a "big" product symbol. – egreg Oct 18 '11 at 19:49
Tx, I didn't realize the typeset mode. – jejuba Oct 18 '11 at 21:03

Has nothing to do with tikZ. You are using the inline mode with $...$ which is typeset in that way. Use
$... \prod\limits_{j:t_j \leq t} ...$