Did you updated the Theme to a Newer Version?
The latest Version includes the update for the different section and widgets to load smaller image sizes which helps faster page load.
Actually there is not option for these setting. They are set by default. The image sizes are set accordingly depending on layout. If you really have high resolution images then you can decrease the image size. There are many free online site and applications, for now we recommend you to use Compressor.io to decrease the image sizes and it won’t affect the image quality.