Image only being made smaller by includegraphics [scale...]

Working in rmarkdown. This is my YAML:

output:
  pdf_document:
    latex_engine: xelatex

I have one image that I would like to increase the size of as the font inside is small:

begin{figure}[h]
begin{center}
includegraphics{no_contraction.jpg}
caption {Regulation of vascular smooth muscle relaxation by Nitric Oxide  Image taken from Griffiths and Evans, 2005}
label{f:no_contraction}
end{center}
end{figure}

This is how it appears:

So I have tried to scale the image:

begin{figure}[h]
begin{center}
includegraphics[scale=1.5]{no_contraction.jpg}
caption {Regulation of vascular smooth muscle relaxation by Nitric Oxide  Image taken from Griffiths and Evans, 2005}
label{f:no_contraction}
end{center}
end{figure}

However, whatever number I put for the scale, it is making the image tiny:

Why is this happening?