Many wp_postmeta entries

I now have tens of thousands of wp_postmeta entries with meta_keys like _01cb4d14798914a514d72f293311cc17 and meta_value of 1. The post_ids are all to images.

Is this plugin responsible, or Regenerate Thumbnails Advanced, or perhaps another image optimisation plugin?

Do you have any published statement on what database changes you make, and what the plugin/s does/do on uninstall?

Thanks.