\textbf{text} without increasing the length of the text

\documentclass{article}
\usepackage{graphicx}
\newsavebox\CBox
\def\textBF#1{\sbox\CBox{#1}\resizebox{\wd\CBox}{\ht\CBox}{\textbf{#1}}}
\parindent=0pt
\begin{document}
foobarbaz y\\
\textbf{foobarbaz} y\\
\textBF{foobarbaz} y

\end{document}

The example for beamer

\documentclass{beamer}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\newsavebox\CBox
\newcommand<>*\textBF[1]{\only#2{\sbox\CBox{#1}\resizebox{\wd\CBox}{\ht\CBox}{\textbf{#1}}}}
\parindent=0pt
\begin{document}
\begin{frame}{foo}
\only<1>{foobarbaz y1} 
\textbf<2->{foobarbaz y2}
\textBF<3>{foobarbaz y3}
\end{frame}

\end{document}

Another (better) possibility is to use the letter spacing option from the package microtype

You can use \fontseries{b}\selectfont to select a non-extended bold font, provided of course that the font family you're using contains such a font (as computer modern does). For comparison, note that \textbf uses \fontseries{bx}, where the xstands for “extended”.

This answer is just for adding overlay support to Herber's command, who should credited for it.

\newcommand<>\textBF[1]{\only#2{\dotextBF}{#1}}
\newcommand\dotextBF[1]{\sbox0{#1}\resizebox{\wd0}{\ht0}{\textbf{#1}}}

...

\textBF<2->{xyz}

"xyz" will be bold from the second screen onwards.

I don't think it's a good way to emphasize things on screen; colors are better.