There are a few options you can consider:

- If you changed the image dimensions used in your site, you can re-create your old/previous K2 item images in the new dimensions using this tool: github.com/getk2/rebuildK2imageCache

- You can delete all images inside /media/k2/items/src/ but if you redesign your site and its required to re-generate the images (e.g. in larger dimensions) you won't have this option.

- Finally, you can run a terminal tool like jpegoptim on the server (if you have the right access) which can optimize JPG images and save you an extra 10% - 30% in total size.