I suggest you check first that these files are all present inside the theme folder. These are not generated but present in the theme package and called from it when necessary. This is an issue related to your server configuration. Please contact your host and make sure all required files are being loaded from it.
You may want to install a cache to make the loading of files faster.
These images are uploaded in high quality so they appear in good in site. You can either optimize these images using any image processing program, or regenerate thumbnails in WordPress dashboard.
The only thing is to change the size of the image manually and re-upload them. We can’t make different thumbnails for any element, if we will do this the page will be very heavy on load.
So the easy way is to change the images size manually and re-upload them.
