"I come across pages with the images approx. 4000x3000 that let the browser do the work of scaling it down to 400x300."
Oh FFS yes! I am continually dismayed by people within our organisation who send enormous emails with full size stupidly high resolution images embedded in docx files which display them at "proper" size. And we are an IT company.
I'm not sure how they manage it because when I have an actual business NEED to send some photos at full size, the bloody mail server refuses to deliver them unless I send them one or two at a time due to attachment size restrictions.