In my document, the widths of captions depend on the widths of the figures:
- For large images captions are large enough

\begin{SCfigure}[][p!]
\begin{tikzpicture}[x=20mm,y=20mm]
\begin{axis}[
std_empty,
width = 70mm,
% height = 60mm,
xmin = -2,
xmax = 2.2,
ymax=1.1,
]
\addplot[ const plot mark mid,
]
coordinates
{ (-2,0)
(-1.5,0.2)
(-1,0.6)
(-0.5,0.7)
(0,0.95)
(0.5,0.8)
(1,0.4)
(1.5,0.3)
(2,0)
};
\addplot[ sharp plot,
]
coordinates
{ (-2,0)
(-1.5,0.2)
(-1,0.6)
(-0.5,0.7)
(0,0.95)
(0.5,0.8)
(1,0.4)
(1.5,0.3)
(2,0)
};
\end{axis}
\end{tikzpicture}
\caption{Poligono di frequenza}
\end{SCfigure}
- but for small ones there is a very little space for captions.

\begin{SCfigure}[][hp!]
\begin{tikzpicture}
\chemfig{
-[:20]
(-[:80]?-[:35,0.75]OH)
-[:-20]
-[:60,0.75]
-[:160]
?-[:-160]
=[:-120,0.75]
}
\end{tikzpicture}
\caption{il norbornen-7-olo presenta un centro stereogenico non chirale}
\end{SCfigure}
Why does this happen?
edit: As asked, a MWE
\documentclass[a5paper,]{book}
\usepackage[rightcaption,]{sidecap}
\usepackage{tikz}
\usepackage{chemfig}
\begin{document}
\begin{SCfigure}[][hp!]
\begin{tikzpicture}
\chemfig{
-[:20]
(-[:80]?-[:35,0.75]OH)
-[:-20]
-[:60,0.75]
-[:160]
?-[:-160]
=[:-120,0.75]
}
\end{tikzpicture}
\caption{il norbornen-7-olo presenta un centro stereogenico non chirale}
\end{SCfigure}
\end{document}
