Actually WordPress reduce quality of image when you upload it via media uploader. WordPress has built in compression for JPG images. Whenever you upload an JPG/JPEG image to WordPress media library, WordPress will automatically compress your images to 90% of the original quality, this is intended to help your pages load faster and keep file sizes smaller.
You can disable image compression and force WordPress to keep maximum quality images with this function.
// Set image quality
function my_image_quality( $quality ) {
return 100;
}
add_filter( 'jpeg_quality', 'my_image_quality' );
add_filter( 'wp_editor_set_quality', 'my_image_quality' );