51.8% of website homepages with images offer only one size of each image, so a phone downloads the same large files as a desktop computer. The web's most visited sites do worse, not better: 60.7% of the top 10,000 and 62.8% of sites ranked 10,000 to 1 million serve one size to every device.

Source: 51.8% of Websites Send Phones the Same Image Files as Desktop Computers - Suff Digital