My suspicion is that some plugin is doing something with generating images on the backend (where it shouldn't be) and causing EWWW to use more memory than normal. I just tested it on an install with over 100,000 images, and I'm using 5MB of memory. I had to check it about 15 times to make sure, and even installed this plugin to verify it: https://wordpress.org/plugins/debug-bar/
The only page I could find which uses more memory is the Bulk Optimize page, but it doesn't even sound like you're getting that far before you hit the OOM errors (out of memory).