tl;dr: It depends.
Long story:
Thumbnail generation exists for several reasons. First of all you can create thumbnail sizes that accomodate to your theme. Images can have just the size you need. Furthermore you can choose the quality setting. In a nutshell: lower quality equals lower file size. And last but not least: You can have retina (2x) versions of thumbnails – with its own quality setting.
What you need to understand is that the chosen optimizer is a fast one that hosting providers usually have installed. The default optimizer has no priority for smallest file sizes possible. (This is usually Guetzli. But it would take like forever to optimize thumbnails with it.) So it depends on what Shopware can use and what is installed on the server.
So what you should do is check your website with different display widths and create thumbnails accordingly – in order to match the actual image sizes in a browser. The thumbnail quality is a tradeoff between image quality and file size – choose what fit your needs. Retina thumbnails are nice to have and can improve the user experience.
You should also check if your theme does the best with these thumbnails. The frontend code needs to use the thumbnails in the right way in order to give browsers the option to choose the correct image size. Sometimes you have to optimize the templates in one way or another to get the most out of the thumbnail settings.
Finally there is a plugin that allows you to integrate webp thumbnails. This can further improve total page transfer size.