Plugin compress the original files in upload time. Remember, compression is not a reversible process of deteriorating the quality of photos. The compression ratio depends on the settings you specify. All thumbnails will be created from a compressed original, so it takes less disk space and it takes less time. The plugin does not compress already loaded original photos and photos that are uploaded through Media manager.
In the Settings you can configure how images are compressed by degrading the quality and/or changing the size of the photo. All images that are uploaded to the website as model attachments are automatically compressed and trimmed to the specified size.