The font of the section is defined by
\newfont{\secfnt}{ptmb8t at 12pt}
So to change it in front of the bibliography use:
\let\secfnt\undefined
\newfont{\secfnt}{ptmb8t at 10pt}
\bibliographystyle{abbrv}
\bibliography{sigproc}
If the bibliography isn't the last section you should group the redefinition:
\documentclass{sig-alternate}
\usepackage{filecontents}
\begin{filecontents}{sigproc.bib}
@book{test,
author={Any Body},
title={Some Title},
year={2012},
puplisher={here}
}
\end{filecontents}
\begin{document}
paper text here!
\cite{test}
\let\secfnt\undefined
\newfont{\secfnt}{ptmb8t at 10pt}
\bibliographystyle{abbrv}
\bibliography{sigproc}
\end{document}
Please provide always Minimal Working examples.
:)
\newfont{\secfnt}{ptmb8t at 12pt}
So you have to change this.