Font-size of aligned multiline subscript

Question

Consider the below single-line and multi-line subscripts.

a_{1,2,3} 

b_{
    \substack{
    1, 2, 3  \\
  + 4, 5, 6
    }
}

I wish to align the 1 and 4 in the multiline b subscript. Merely changing substack to an aligned environment however affects the font-size of the subscript.

b_{
    \begin{aligned}
    & 1, 2, 3  \\
  + & 4, 5, 6
    \end{aligned}
}

Use of aligned has increased the subscript font-size to be that of non-subscripted text. Haphazardly \scriptscriptstyle does not seem to affect the font-size.

b_{
    \scriptscriptstyle
    \begin{aligned}
    \scriptscriptstyle
    & 1, 2, 3  \\
    \scriptscriptstyle
  + & 4, 5, 6
    \end{aligned}
}

How can I create alignment in multi-line subscripts while preserving the subscript font-size?

Answer 1

Insert a \phantom component that matches the component causing the misalignment.

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align*}
  & a_{1,2,3} \\
  & b_{
    \substack{
      \phantom{+} 1, 2, 3  \\
               +  4, 5, 6
    }}
\end{align*}

\end{document}