Indeed the behavior you've described is the default in revtex4-1.
This isn't "in effect", as you said, just because you're loading the caption package, which is incompatible with that class.
So, don't load either caption nor subcaption and you will have your captions in a \small font and justified.
MWE
\documentclass[reprint,aps,prl,twocolumn ,groupedaddress,nobibnotes]{revtex4-1}
\usepackage[demo]{graphicx}
\begin{document}
\begin{figure}
\includegraphics[width=.9\linewidth]{circuits.png}
\caption{This is a long sentence to test whether the caption is justified
or not. Unfortunately it is not.}
\end{figure}
\end{document}
Output

EDIT (in response to the OP's comment)
minipages are not incompatible with revtex4-1.
Here's an example of how to have two images side by side, in one column and in two columns mode.
\documentclass[reprint,aps,prl,twocolumn ,groupedaddress,nobibnotes]{revtex4-1}
\usepackage[demo]{graphicx}
\usepackage{lipsum}
\begin{document}
\begin{figure}
\begin{minipage}[t]{0.48\linewidth}
\includegraphics[width=.9\linewidth]{circuits.png}
\caption{This is a long sentence to test whether the caption is justified
or not. Unfortunately it is not.}
\end{minipage}\hfill%
\begin{minipage}[t]{0.48\linewidth}
\includegraphics[width=.9\linewidth]{circuits.png}
\caption{Another figure.}
\end{minipage}%
\end{figure}
\begin{figure*}
\begin{minipage}[t]{0.48\linewidth}
\includegraphics[width=.9\linewidth]{circuits.png}
\caption{This is a long sentence to test whether the caption is justified
or not. Unfortunately it is not.}
\end{minipage}\hfill%
\begin{minipage}[t]{0.48\linewidth}
\includegraphics[width=.9\linewidth]{circuits.png}
\caption{Another figure.}
\end{minipage}%
\end{figure*}
\end{document}
Output (two images in the same column)

Output (two images spanning two columns)

Original versioncomment at the top of the.clsfile:-) – David Carlisle Jun 20 '19 at 17:59