OK, so I talked with the team at Imgix and we managed to figure it out that it's a combination of the compression and enchance to return a high quality image at a low file size for PNGs.
So if you view the image below, which has ?auto=enhance,compress,format
appended, it comes back at 12kb:
https://images.prismic.io/example-repo/70aaf798-51e0-4442-a1ac-0b30cab56167_DS_Change_Password.PNG?auto=enhance,compress,format
So if you return you default Prismic URL, which has ?auto=compress,format
in your project and append enhance
to the URL string, it should look like this:
newUrl =${img}&auto=enhance,compress,format
This should return a sharp image with a small file size. Let me know if this works for you.
Thanks.