I believe what they're saying is that the thumbnails Wordpress generates may be larger in file size because they're not rigorously optimized like your original image may be.

There's browser plugins that will loop through your entire media library and ensure they're optimized. TinyPNG and Smush are well-known ones.

Yes, that's exactly what I meant. I optimise my uploaded images highly and when WP created the various sizes, the smaller pixel sizes often end up heavier in kb than the original.

So I have to optimise them all, now it is done automatically.

I'd like to see that. That would mean either WP is adding meta to the image file (which I'm pretty sure it doesn't do), or WP is making a lower quality jpg in higher which - without a special AI-y algorithm - isn't going to yield true higher res.

I have definitely seen that happen with the PHP blog engine that runs my blog, either with the GD image functions or with the imagemagick integration. Probably something like: Take an optimized image, like an indexed png, load it as raw image data, scale it down, save it in the same format but not optimized. The Wordpress code looks similar.

I've had this happen when uploading a highly optimized image (manually optimized with Squoosh) 2560px wide, then WP generating a 1920px wide version and the KB was larger. I believe I was using the free version of Smush at the time and had to change. Now using reSmush.it.