Actually there is a more legal way to do this using captions package option width.
For global effect
\usepackage[width=.75\textwidth]{caption}
For local effect only in current environment:
\usepackage{caption}
\captionsetup{width=.75\textwidth}
More info in caption package doc:
https://www.ctan.org/pkg/caption?lang=en
http://mirrors.ctan.org/macros/latex/contrib/caption/caption-eng.pdf (direct link to PDF subject to change)