Sixty-two percent of the weight of the web is images, and we’re serving more image bytes every day. That would be peachy if all of those bytes were being put to good use. But on small or low-resolution screens, most of that data is waste.
Responsive images
To complicate it (and sometimes, balance it), the small screens are usually quite high resolution. iPhone 6 Plus has the same resolution as the 21.5-inch iMac.
Of course if all desktop displays were @2x that wouldn’t be the case.
don’t forget to add dumb web designers and/or journalists to the equation — I’ve seen a lot of popular sites (such as Apple Insider) using megabytes of PNG images in their stories. often then content are photos, where JPEG compression would be a better choice. on the desktop you notice these slowly loaded pages less, but on mobile it really stands out.
“back in my day” we knew how to optimize images for the web based on content.