How to display sine squared more compactly

I can reproduce your picture only if I add \left and \right in front of the parentheses.

So just don't use them.

\documentclass{article}
\begin{document}
$\sin^{2}\theta$

$\sin^{2}(\theta)$

$\sin^{2}\left(\theta\right)$
\end{document}

By the way, without parentheses it's even better.