fitting text into "size" of convert standalone class

I'm generating a .png image using the standalone class.

\documentclass[preview, border=0.5 pt, convert={size=650x650,outext=.png}]{standalone}\usepackage{amsfonts}\usepackage{mathtools}\usepackage{amsmath}\usepackage{amssymb}\usepackage{braket}\usepackage[utf8]{inputenc}\begin{document}\begin{equation*}p(x) = \sum_{i=0}^n a_ix^i, \quad q(x) = \sum_{j=0}^m b_ix^j, \quad m \ge n\end{equation*}\end{document}

and compiling with pdflatex --shell-escape formula.tex

The problem is that there is a lot of unused space in the image as you can see below compared to text,

If I try to shrink the resolution of the x dimension it reduces as well the font size. Is there a way to optimize resolution vs font size?