[Tex/LaTex] How to make underbrace texts have the normal-text size

Question

Situation

I am using \underbrace with \substack for multi-line underbracing. The problem is that the text under the brace is small relative to the normal text. I want to make the size of texts under braces the same to that of normal texts.

Example

Here is an example:

\implies \underbrace{x(t) = z(t)}_{\substack{x(t)=y(t)\\y(t)=z(t)}}

As you see, the formulas under the brace are small relative to the normal formulas. It seems like their size is subscripts' size.
I want something like this:

Notes

Commands like \displaystyle, \textstyle, \normalfont in the underbrace did not solve my problem.

Answer 1

\scriptstyle is hardcoded into environment subarray, which is used by \substack. As workaround, \scriptstyle can be locally redefined as \textstyle:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\[
  \implies \underbrace{x(t) = z(t)}_{%
    \let\scriptstyle\textstyle
    \substack{x(t)=y(t)\\y(t)=z(t)}}
\]
\end{document}

A cleaner result can be achieved by using environment array together with \textstyle. This fixes also the too narrow line spacing of \substack:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\[
  \implies \underbrace{x(t) = z(t)}_{%
    \textstyle
    \begin{array}{c}
      x(t)=y(t)\\
      y(t)=z(t)
    \end{array}
  }
\]
\end{document}

And a third variant using gathered

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\[
  \implies \underbrace{x(t) = z(t)}_{%
    \textstyle
    \begin{gathered}
      x(t) = y(t)\\
      y(t) = z(t)
    \end{gathered}
  }
\]
\end{document}