Image version on Squarespace is larger than upload version. Why?

I am trying to tackle a page speed issue by redesigning and resizing images on my portfolio website. When I upload a compressed 2000px wide JPG image with 485kb and then right click, download, the image it is still 2000px wide, but all of a sudden it’s 618kb large. Why does SP enlarge the file by 30%? How do I please the Google-SEO-Gods, who are constantly complaining about my image size? 

Squarespace automatically compresses most images when you upload them; however, like in this circumstance, I have found where SS will enlarge an image.

Try compressing the image further using https://imagecompressor.com/ and see how SS treats it then - I have found before that if I compress it a bit more, SS doesn't enlarge it upon upload.

If this doesn't work, convert the image into a compressed PNG and try again.

I think the reason why this happens is that Squarespace automatically creates set widths for images. Based upon this article, https://support.squarespace.com/hc/en-us/articles/206542517-Formatting-your-images-for-display-on-the-web?platform=v6&websiteId=6214c6576b6195311edf4f9b, SS could be increasing the width of your image to 2500 pixels. However, I'm still working out precisely what happens here and figuring out if my plugin can fix it.

Hopefully that helps