# floatrow and capbesidewidth for side caption

I am having some weird problem using fcapside in floatrow where I can't get the side caption to auto-calculate its width based on my figure. Instead, the side caption width seems to expand with the image. I set the capbesidewidth to something else in the preamble so I explicitly set it back to sidefil (or none) for the figure, but it does this even if I never set it anywhere. I can't figure out why this is different from the manual where the caption width is auotmatically calculated to occupy the rest of the space.

\documentclass[11pt,twoside]{report}

\usepackage{graphicx}
%\usepackage{caption}
\usepackage{floatrow}
\DeclareFloatVCode{largevskip} {\vskip 20pt}
%\floatsetup[figure]{a bunch of other settings here that I overwrite below}
\captionsetup[figure]{font=small}
\usepackage{subfig}

\begin{document}

\begin{figure}[htbp]
%\centering
\ffigbox[\textwidth]
{\begin{subfloatrow}
\fcapside[\FBwidth]
{\setlength\fboxsep{4pt} \fbox
{\includegraphics[width=0.5\textwidth]{example-image-a}}}
{\caption{Hello I am a caption and my width seems to be approximately the same as that of the image next to me, which is 0.5 times the text width.}}
\end{subfloatrow}
\begin{subfloatrow}
\fcapside[\FBwidth]
{\setlength\fboxsep{4pt} \fbox
{\includegraphics[width=0.65\textwidth]{example-image-a}}}
{\caption{Hello I am a caption and my width also seems to be approximately the same as that of the image next to me, which is 0.65 times the text width.}}
\end{subfloatrow}}
{\caption{I am a centered caption for the full figure.}}
\end{figure}

\end{document}