LazySizes is the more common of the two in our index: it runs on 3,230 websites against 309 for Autoptimize, and 137 sites use both. Counts come from real scans, not vendor claims.
Autoptimize is a WordPress plugin that optimises website performance by aggregating, minifying, and compressing HTML, CSS, and JavaScript files.
LazySizes is a JavaScript library used to delay the loading of images (iframes, scripts, etc) until they come into view.
137 sites run both Autoptimize and LazySizes. That is 44% of the sites using Autoptimize, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.