Switching from equation to align changes horizontal width

Question

I am puzzled by why changing from an equation environment to an align environment should change the horizontal width of my equation. Below I provide an example. Sorry, it is not a minimal working example, since it is hard to replicate this issue. This is the PDF that I get by running the code, see here.

Can anyone tell me how to fix this issue so that equations (2) and (3) are identical to equation (1)?

\documentclass[11 pt]{article}

\usepackage{amsmath}
\usepackage{geometry}
\geometry{
  a4paper,
  includeheadfoot,
  hmargin = 2.6cm,
  vmargin = 2cm
}

\newcommand{\al}{\alpha}
\newcommand{\be}{\beta}
\newcommand{\ipos}[1]{i_+(#1)}

\begin{document}

\begin{equation}%
\bigl( \al^2 + \be^2(1 + s)\rho - \al \be(1 + s)(\rho + 1) \bigr) \ipos{\al,\be,r} + \al \be s \ipos{\al,\be,r+1} = 0, \quad r = 0,\ldots,s-2,
\end{equation}%

\begin{align}%
\bigl( \al^2 + \be^2(1 + s)\rho - \al \be(1 + s)(\rho + 1) \bigr) \ipos{\al,\be,r} + \al \be s \ipos{\al,\be,r+1} = 0, \quad r = 0,\ldots,s-2,
\end{align}%

\begin{align}%
\bigl( \al^2 + \be^2(1 + s)\rho - \al \be(1 + s)(\rho + 1) \bigr) \ipos{\al,\be,r} + \al \be s \ipos{\al,\be,r+1} &= 0, \quad r = 0,\ldots,s-2, \\
a &= 0.
\end{align}%

\end{document}

Answer 1

The problem you are seeing here is that equation and align do use different approaches when a formula is getting too long (what is the case here).

equation is trying to compensate that mistake by replacing the normal inter-symbol-distance by a 'squeezable' distance (pink in my image below). align does not use this trick.

I give two examples with different text-width. The first is just wide enough, the second is too narrow. I hope my images are understandable (the blue marks define possible hyphenation points here. Can't switch that off. But the width of both aligns kept the same).

% arara: lualatex

\documentclass{article}
\usepackage{amsmath}
\usepackage{lua-visual-debug}
\usepackage[textwidth=1.4cm]{geometry}

\begin{document}    
    \begin{equation*}
        a + b + c
    \end{equation*} 
    \begin{align*}
        a + b + c
    \end{align*}    
    \newgeometry{textwidth=1.3cm}%
    \begin{equation*}
    a + b + c
    \end{equation*} 
    \begin{align*}
    a + b + c
    \end{align*}
\end{document}

Text-width = 1.4 cm:

Text-width = 1.3 cm:

Text-width = 1 cm:

---

PS: I used the starred version of each, as the labelling puts even more stuff to the problem. Without the stars, the behaviour is just the same.

PPS: In order to give a solution to your problem: shorten your formula or use an align even for the one-line version!