Does it create new thumbnails that didn`t exist before?

Short answer yes!

It will re-generate thumbnails of all registered image sizes (https://developer.www.ads-software.com/reference/functions/add_image_size/) for each image in the media library.

and remove the old size thumbs?

@b_dark It will recreate all thumbnails and therefore overwrite existing thumbnails.

I’m pretty sure it will NOT remove old thumbnails for outdated sizes. Those would have to be manually cleaned up if you were worried about the space they take.

@floq-design i believe in a plugin with 1+m downloads its good idea to add a option to remove the old thumbnails and a option to recreate only the featured image posts ??

@jonez I agree – the plugin does not remove thumbnails for outdated sizes. My previous comment was not correct.

@b_dark It’s worth suggesting these feature to the developers. Removing old thumbnails could cause issues if an old image size is deleted that has been embedded into a post/page.

I think, I have same issue… My site have 8700 pictures. With Woocommerce and Divi Theme it is next 11 thumbnails per picture (it’s 95700 thumbnails !). I disabled medium and large sizes and other with remove_image_size(). But plugin still regenerating and creating all 11 thumbnails ??