“One-size-smaller than the accompanying font size” command [duplicate]

I want to make a subtitle for a chapter, having \Large as the chapter's title font size; so I'd write it as

\documentclass{book}
\begin{document}
\tableofcontents
\chapter{Title \\ {\large Subtitle}}
\end{document}


This would make the subtitle's font size look good where the title is, but look horrific in the toc. So I'm asking, is there any kind of command or workaround so that will decrease subtitle's font size by one size, in both occasions? From \Large to \large in the chapter and from \normalsize to \small in the toc.

The easy fix is to use the optional "short title" parameter of the \chapter command:
\documentclass{book}

For the general solution to font sizes relative to the current one you can use the relsize package (see here for a related question).
Personally I would not be very happy introducing so much style-dependent formatting in titles; if this situation is not an exception but the norm I would define my own sectioning style, maybe with the help of the titlesec package and custom macros for title+subtitle chapters.