WordPress Resized Images (to smaller dimensions) larger than source image

I’m not sure if this was always a thing but I don’t recall it happening with older versions of wordpress.

I’ve uploaded a 900px width image to wordpress that’s 350kb.

Inserting this image into a page, I chose to resize it to default ‘large image’ size which is 800px width.

When I save this image to my computer from the page it’s published on, I notice the 800px resized image is now 1000kb in file size.

How can wordpress resize the dimensions of the source image to 100px width smaller yet the file size somehow increases, and from 350kb to 1000kb — about 3x!.

How exactly does that work? And why?