Btukfyl

A MWE is as follows:

documentclass{amsart}



begin{document}

Huge

We have

[

f(x)=

begin{cases}

2x^{2018}+9&x<2018\

3x+2018&xgeq 2018.

end{cases}

]

And we also have

[

left(

begin{array}{cc}

 x   &m  \

 y   &n  \

 x   &m  \

 y   &n

end{array}

right).

]



end{document}

As you see in the PDF, the large brace and parenthesis are very thick.

Then my question is: How to make large brace and parenthesis be thinner? Any help is welcome!

You could (ab)use scalebox.

documentclass{amsart}

usepackage{graphicx}

begin{document}

Huge

We have

[

f(x)=

scalebox{0.5}[1]{$displaystyle

left{scalebox{2}[1]{$displaystylebegin{array}{ll}

2x^{2018}+9&x<2018\

3x+2018&xgeq 2018.

end{array}$}right.$}

]

And we also have

[

scalebox{0.5}[1]{$displaystyle

left(scalebox{2}[1]{$displaystyle

begin{array}{cc}

 x   &m  \

 y   &n  \

 x   &m  \

 y   &n

end{array}$}

right)$}.

]

end{document}

If you want to use that very often, you may define your own environments for that.

documentclass{amsart}

usepackage{graphicx}

usepackage{environ}

NewEnviron{thincases}{scalebox{0.5}[1]{$displaystyle

left{scalebox{2}[1]{setlength{arraycolsep}{6pt}% <- I did not look up the "correct" value

$displaystylebegin{array}{ll}

BODY

end{array}$}right.$}}%}

NewEnviron{thinpmatrix}{scalebox{0.5}[1]{$displaystyle

left(scalebox{2}[1]{$displaystyle

begin{matrix}

BODY

end{matrix}$}

right)$}}

begin{document}

Huge

We have

[

f(x)=

begin{thincases}

2x^{2018}+9&x<2018\

3x+2018&xgeq 2018.

end{thincases}

]

And we also have

[

begin{thinpmatrix}

 x   &m  \

 y   &n  \

 x   &m  \

 y   &n

end{thinpmatrix}

]

end{document}

Ideally, you should choose a font that gives the brace dimensions you seek. And so, I would not recommend this workaround for general use, but the scalerel package can scale glyphs in a width-limited fashion:

documentclass{amsart}

usepackage{scalerel}

begin{document}

Huge

We have

[

f(x)=

scaleleftright[13pt]{biggl{}{

begin{aligned}

2x^{2018}+9&x<2018\

3x+2018&xgeq 2018.

end{aligned}

}{.}

]

And we also have

[

scaleleftright[13pt]{biggl(}{

begin{array}{cc}

 x   &m  \

 y   &n  \

 x   &m  \

 y   &n

end{array}}{biggr)}

.

]

end{document}

Alternately, they can stretch them in an aspect-ratio-limited fashion.

As both of the previously posted answers have already pointed out, by far the most elegant way to obtain thinner curly braces and round parentheses is to switch to a textfont/mathfont combination whose curly braces and round parentheses are thinner than those of Computer Modern.

The following four screenshot show the varying looks produced by Computer Modern, a Times Roman clone, and two Palatino clones. To my eye, the tall curly brace and the tall round parentheses produced by the newtxmath package are quite a bit thinner than those of the other fonts.

And here's the code that gives rise to the preceding screenshots.

documentclass{amsart}

%usepackage{newtxtext,newtxmath} % Times Roman clone

%usepackage{newpxtext,newpxmath} % Palatino clone

usepackage{kpfonts} % another Palatino clone

begin{document}

%Computer Modern (default)

%newtxtext/newtxmath

%newpxtext/newpxmath

kpfonts

begin{gather*}

f(x)=

begin{cases}

  2x^{2018}+9 & x<2018    \

  3x+2018     & xgeq 2018.

end{cases}\

begin{pmatrix}

  x & m \ y & n \ x & m \ y & n

end{pmatrix}

end{gather*}

end{document}