Raise the number of comments retrieved in comment feeds from the default (10) to 100:

In PHP 5.2 or earlier:

/**
 * Filter the LIMIT clause on comment feed queries.
 *
 * @param string   $limit    LIMIT clause for the comment feed query.
 * @param WP_Query $instance WP_Query instance, passed by reference.
 * @return string (maybe) Filtered comment feed query LIMIT clause.
function wpdocs_raise_comment_feed_limit( $limit, &$instance ) {
	return 'LIMIT 100';
}
add_filter( 'comment_feed_limits', 'wpdocs_raise_comment_feed_limits', 10, 2 );

In PHP 5.3 or later:

// Filter the LIMIT clause on comment feed queries to increase the number from 10 to 100.
add_filter( 'comment_feed_limits', function( $a ) { return "LIMIT 100"; } );