Good morning @tamlyne,
Thank you for contacting us.
and sorry for the inconvenience.
When you upload an image to your website, WordPress will create different thumbnail versions of it (see Settings > Media). The plugin will compress each of these sizes, so when you have 100 images and 5 different sizes you will do 500 compressions.
If you wish to manage which image sizes get generated and compressed by the plugin, in your WordPress dashboard go to Settings > Compress JPEG & PNG images and uncheck any image sizes you do not wish to use.
If you would like us to look more into it,
please contact us via email at support@tinypng.com.