It’s the three images – one on each of the main pages. I just resized them all and uploaded new versions – maximum resolutions of 640×480. I deleted the Adaptive Images cache and the existing images from the site via FTP, also.
The one on the main page, for example: https://tutorsaurusrex.com/wp-content/uploads/2018/04/home.png should be 443×480 but my phone is still getting a 1000px version – as far as I can tell from looking at it as it takes up 3/4 of the screen.
I don’t know if there is some caching or CDN I don’t know about involved but I’m about to restart the web app and see if that makes a difference.