Feynman diagram using tikz-feynman package has wrong size

Question

I have a simple question comming the following code:

\documentclass{article}
% lualatex
\usepackage{tikz-feynman}

\begin{document}
\begin{figure}[h!]
\centering

\feynmandiagram [inline=(b), horizontal=a to b] {
a --[fermion] b [dot]  -- [blue, gluon] c, 
b  -- [fermion] d
};
\qquad
\feynmandiagram [inline=(b), horizontal=a to b] {
a --[blue, gluon] b [dot]  -- [ghost] c, 
b  -- [ghost] d
};
\qquad
\feynmandiagram [inline=(b), horizontal=a to b] {
a --[blue, gluon] b [dot]  -- [blue, gluon] c, 
b  -- [blue, gluon] d
};
\qquad
\feynmandiagram [inline=(b), horizontal=a to d] {
a --[blue, gluon] b [dot]  -- [blue, gluon] c, 
b  -- [blue, gluon] d,
b  -- [blue, gluon] e,
};

\end{figure}
\end{document}

The forth diagram is a litle bit bigger than the others, as can be seen here:

How should I proceed in order to make all the diagrams have the same size?

Answer 1

I wouldn't use tools like \resizebox or similar to scale the diagram. This will change the line width and size of the dot and will make your thesis look like a ransom letter.

Instead you could nudge the coordinates a bit to move in the right direction:

% !TeX TS-program = lualatex

\documentclass{article}
% lualatex
\usepackage{tikz-feynman}

\begin{document}
\begin{figure}[h!]
\centering

\feynmandiagram [inline=(b), horizontal=a to b] {
a --[fermion] b [dot]  -- [blue, gluon] c, 
b  -- [fermion] d
};
\qquad
\feynmandiagram [inline=(b), horizontal=a to b] {
a --[blue, gluon] b [dot]  -- [ghost] c, 
b  -- [ghost] d
};
\qquad
\feynmandiagram [inline=(b), horizontal=a to b] {
a --[blue, gluon] b [dot]  -- [blue, gluon] c, 
b  -- [blue, gluon] d
};
\qquad
\feynmandiagram [inline=(b), horizontal=a to d] {
a[shift={(0.1cm,-0.1cm)}] --[blue, gluon] b [dot]  -- [blue, gluon] c[shift={(-0.2cm,0.2cm)}], 
b  -- [blue, gluon] d[shift={(-0.2cm,-0.1cm)}],
b  -- [blue, gluon] e[shift={(0.1cm,0.2cm)}],
};

\end{figure}
\end{document}

Alternatively use the tikz scale option, which will leave the line width, dot size etc. intact:

% !TeX TS-program = lualatex

\documentclass{article}
% lualatex
\usepackage{tikz-feynman}

\begin{document}
\begin{figure}[h!]
\centering

\feynmandiagram [inline=(b), horizontal=a to b] {
a --[fermion] b [dot]  -- [blue, gluon] c, 
b  -- [fermion] d
};
\qquad
\feynmandiagram [inline=(b), horizontal=a to b] {
a --[blue, gluon] b [dot]  -- [ghost] c, 
b  -- [ghost] d
};
\qquad
\feynmandiagram [inline=(b), horizontal=a to b] {
a --[blue, gluon] b [dot]  -- [blue, gluon] c, 
b  -- [blue, gluon] d
};
\qquad
\feynmandiagram [inline=(b), horizontal=a to d,scale=0.95] {
a --[blue, gluon] b [dot]  -- [blue, gluon] c, 
b  -- [blue, gluon] d,
b  -- [blue, gluon] e,
};

\end{figure}
\end{document}

Answer 2

You can use scalerel package with the option \scaleobj to zoom in or out. The value is .87.

\documentclass{article}
% lualatex
\usepackage{tikz-feynman}
\usepackage{scalerel}
\begin{document}
\begin{figure}[h!]
\centering

\feynmandiagram [inline=(b), horizontal=a to b] {
a --[fermion] b [dot]  -- [blue, gluon] c, 
b  -- [fermion] d
};
\qquad
\feynmandiagram [inline=(b), horizontal=a to b] {
a --[blue, gluon] b [dot]  -- [ghost] c, 
b  -- [ghost] d
};
\qquad
\feynmandiagram [inline=(b), horizontal=a to b] {
a --[blue, gluon] b [dot]  -- [blue, gluon] c, 
b  -- [blue, gluon] d
};
\qquad
\scaleobj{.87}{\feynmandiagram [inline=(b), horizontal=a to d] {
a --[blue, gluon] b [dot]  -- [blue, gluon] c, 
b  -- [blue, gluon] d,
b  -- [blue, gluon] e,
};}

\end{figure}
\end{document}