How to reduce thumbnails?

First – these are snippets for child-theme functions.php!

to reduce the compression level of calculated images :

/**** abstellen der von Enfold auf 100 gesetzten jpeg Qualität  *****/
add_filter("avf_jpeg_quality", "avf_set_quality_mod", 9999, 1);
add_filter("avf_wp_editor_set_quality", "avf_set_quality_mod", 9999, 1);
function avf_set_quality_mod($quality) { $quality = 55; return $quality;}

A) for WordPress image size formats

// Remove unneeded WP image sizes
// Remove default image sizes here. 
function prefix_remove_default_images( $sizes ) {
// do NOT remove small and medium sizes, they are used in backend Media Library!
// unset( $sizes['small']); // 150px
// unset( $sizes['medium']); // 300px
 unset( $sizes['large']); // 1024px
 unset( $sizes['medium_large']); // 768px
 return $sizes;
}
add_filter( 'intermediate_image_sizes_advanced', 'prefix_remove_default_images' );

B) for Enfold image size formats

function remove_enfold_image_sizes() {
// do NOT remove widget size, is used in backend portfolio items!
// remove_image_size('widget');
  remove_image_size('square');
  remove_image_size('featured');
  remove_image_size('featured_large');
  remove_image_size('portfolio');
  remove_image_size('portfolio_small');
  remove_image_size('gallery');
  remove_image_size('magazine');
  remove_image_size('masonry');
  remove_image_size('entry_without_sidebar');
  remove_image_size('entry_with_sidebar');
  remove_image_size('shop_thumbnail');
  remove_image_size('shop_catalog');
  remove_image_size('shop_single'); 
  remove_image_size('shop_gallery_thumbnail');
}
add_action('init', 'remove_enfold_image_sizes');

After inserting these snippets you had to recalculate thumbnails – use a plugin for that
i prefer: regenerate-thumbnails-advanced