@JSWolf - this shows they are structurally identical:
I don't know why Modify EPUB shrunk the cover from 1200x1920 to 741x1186 and Polish didn't?
I know the plugin can do more than Polish, I use it to remove the non Dublin Core metadata… but if all I wanted was to inject covers in bulk I would use Polish… avoids having to install/update a plugin, and its faster.
BR