Aligned equation in underbrace is too big

Question

Whenever you use underbrace in maths mode

\underbrace{x}_{=y}

y is in a smaller size than x.

But if I use an entire aligned equation

\underbrace{x}_{
   \begin{aligned}[t]
      &=y_1 \\
      &=y_2
   \end{aligned}
}

the aligned content is in normal size. This looks way too big. Is there a way to reduce the size again? \scriptstyle has no effect. \scriptsize does what I want, but it throws a warning that it is not supposed to be used in maths mode.

Answer 1

You probably mean

\underbrace{x}_{y}

You can get what you want with subarray.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\[
\underbrace{x}_{\begin{subarray}{l} =y_1 \\ =y_2 \end{subarray}}
\]

\end{document}