Long lists of short bullet points look better when put besides using multiple columns. With top-level itemizations, this works easily with the environment columns. But when I try to use it inside a sublist
\documentclass{beamer}
\begin{document}
\begin{frame}
\begin{itemize}
\item a top item
\begin{itemize}
\begin{columns}
\begin{column}{.5\textwidth}
\item a sub item
\end{column}
\begin{column}{.5\textwidth}
\item another sub item
\end{column}
\end{columns}
\end{itemize}
\end{itemize}
\end{frame}
\end{document}

it results in too broad items. As the sub-items also have the same font size as the one above, seemingly the itemization depth does not propagate into the columns.
Putting further itemize-environments into the columns did not work either. The generated document does not change.
\documentclass{beamer}
\begin{document}
\begin{frame}
\begin{itemize}
\item a top item
\begin{columns}
\begin{column}{.5\textwidth}
\begin{itemize}
\item a sub item
\end{itemize}
\end{column}
\begin{column}{.5\textwidth}
\begin{itemize}
\item another sub item
\end{itemize}
\end{column}
\end{columns}
\end{itemize}
\end{frame}
\end{document}
How can I columnize subitems in a single-columned list?
