Basically you have to set the beamer templates section in toc and subsection in toc associated to the circle option; the original definitions can be found in the file beamerbaseauxtemplates.sty. Here are the original definitions for reference:
\defbeamertemplate{section in toc}{circle}
{\leavevmode\leftskip=2ex%
\llap{%
\usebeamerfont*{section number projected}%
\usebeamercolor{section number projected}%
\begin{pgfpicture}{-1ex}{0ex}{1ex}{2ex}
\color{bg}
\pgfpathcircle{\pgfpoint{0pt}{.75ex}}{1.2ex}
\pgfusepath{fill}
\pgftext[base]{\color{fg}\inserttocsectionnumber}
\end{pgfpicture}\kern1.25ex%
}%
\inserttocsection\par}
\defbeamertemplate{subsection in toc}{circle}
{\leavevmode\leftskip=2em\inserttocsubsection\par}
The following sample document shows a possible redefinition increasing the radius of the circle used for sections and using a \bullet for subsections:
\documentclass{beamer}
\mode<presentation>
{
\usetheme{Warsaw}
\setbeamercovered{transparent}
}
\setbeamertemplate{section in toc}
{\leavevmode\leftskip=2ex%
\llap{%
\usebeamerfont*{section number projected}%
\usebeamercolor{section number projected}%
\begin{pgfpicture}{-1ex}{0ex}{1ex}{2ex}
\color{bg}
\pgfpathcircle{\pgfpoint{0pt}{.75ex}}{1.7ex}
\pgfusepath{fill}
\pgftext[base]{\color{fg}\inserttocsectionnumber}
\end{pgfpicture}\kern1.25ex%
}%
\inserttocsection\par}
\setbeamertemplate{subsection in toc}
{\leavevmode\leftskip=2em$\bullet$\hskip1em\inserttocsubsection\par}
\begin{document}
\begin{frame}{Outline}
\tableofcontents
\end{frame}
\section{Introduction}
\subsection{First Subsection Name}
\section{Test}
\subsection{Second Subsection}
\begin{frame}{Make Titles Informative.}
\end{frame}
\begin{frame}{Make Titles Informative.}
\end{frame}
\end{document}

If you want to imitate the ball style for sections, but using \bullet for subsections, the necessary redefinitions will look something like:
\makeatletter
\setbeamertemplate{section in toc}
{\leavevmode\leftskip=2.75ex%
\llap{%
\normalsize%
\begin{pgfpicture}{-1ex}{-0.7ex}{1ex}{1ex}
\pgftext{\beamer@usesphere{section number projected}{tocsphere}}
\pgftext{%
\usebeamerfont*{section number projected}%
\usebeamercolor{section number projected}%
\color{fg!90!bg}%
\inserttocsectionnumber}
\end{pgfpicture}%
\kern1.25ex}%
\inserttocsection\par
}
\setbeamertemplate{subsection in toc}
{\leavevmode\leftskip=2em$\bullet$\hskip1em\inserttocsubsection\par}
\makeatother