SubFieldImage makes image file size larger instead of smaller during resize

[dgarciabriseno]

While testing PUNCH, I noticed that when executing downloadImage on a PUNCH image, this code

api/src/Image/SubFieldImage.php

Lines 340 to 345 in e5980c9

	$coloredImage->resizeImage(
	round($this->subfieldRelWidth),
	round($this->subfieldRelHeight),
	$this->imageOptions['rescale'],
	$rescaleBlurFactor
	);

increases the file size from ~5MB to ~14MB

The resulting image looks the same before and after this line, they are both square 4k images.
We should figure out what's going on here.