although we can apply some CSS to force images to be all the same size, this is not good for performance as it means the browser cannot select src-set images based on the current screen/viewport size.
The best solution is to ensure all your featured images are the same size / aspect ratio before uploading them. Using a desktop application such as Photoshop/Affinity Photo ( or others ) to crop the images first
In the autoptimize settings disable all of the CSS optimizations. Check the site. If the issue has gone then you know its the CSS settings. Then enable one setting at a time to see which one is causing the problem. It is most like the aggregate inline CSS option.