scale height of tree and split headlines with fontsize of `section in head/foot`

[samcarter]

Test document:

\documentclass{beamer}

\usetheme{Warsaw}
%\useoutertheme{tree}

\title{text}

\setbeamerfont{section in head/foot}{size=\huge}
\setbeamerfont{subsection in head/foot}{size=\normalsize}

\begin{document}

\section{section}
\subsection{subsection}
\begin{frame}
    abc
\end{frame} 

\end{document}