How to modify the style of Latex brackets?

Question

For example, make the following braces thinner.

Answer 1

You have not given any clues about your current input, but usually you can select a delimiter from a smaller font which will extend to the same height, but be thinner.

\documentclass{article}

\usepackage{amsmath}

\begin{document}

xxxxx $
\left\{
  \begin{aligned}
    x+y&=0\\
    x+2y&=0
  \end{aligned}
\right.
$
xxx

\bigskip


xxxxx $\scriptscriptstyle
\left\{
  \begin{aligned}
    x+y&=0\\
    x+2y&=0
  \end{aligned}
\right.
$
xxx

\end{document}

Answer 2

Use \scriptstyle and \scriptscriptstyle to change the font-size.

\documentclass[UTF8, 12pt]{article}

\usepackage{amsmath}
\setlength{\parindent}{0pt}
\renewcommand{\baselinestretch}{1.5}
\DeclareMathSizes{12}{10}{7}{5} % default size
% \textstyle, \displaystyle, \scriptstyle, \scriptscriptstyle

\begin{document}
1. XXXXXXXX $
\left\{
\begin{aligned}
    x + y - 2z - 1 = 0  \\
    x + 2y - z + 1 = 0
\end{aligned}
\right.
$ XXXXXXXX.

2. XXXXXXXX $
\displaystyle % 10pt
\left\{
\begin{aligned}
    x + y - 2z - 1 = 0  \\
    x + 2y - z + 1 = 0
\end{aligned}
\right.
$ XXXXXXXX.

3. XXXXXXXX $
\scriptstyle % 7pt
\left\{
\begin{aligned}
    x + y - 2z - 1 = 0  \\
    x + 2y - z + 1 = 0
\end{aligned}
\right.
$ XXXXXXXX.

4. XXXXXXXX $
\scriptscriptstyle % 5pt
\left\{
\begin{aligned}
    x + y - 2z - 1 = 0  \\
    x + 2y - z + 1 = 0
\end{aligned}
\right.
$ XXXXXXXX.

\end{document}