Changing the posts per page on first page

adjust_offset_pagination.php

add_action( 'pre_get_posts', 'query_offset', 1 );
function query_offset( &$query ) {

    // Before anything else, make sure this is the right query...
    if ( ! ( $query->is_home() || is_main_query() ) || is_admin() ) {
        return;
    }

    // First, define your desired offset...
    $offset = -1;

    // Next, determine how many posts per page you want (we'll use WordPress's settings)
    $ppp = get_option( 'posts_per_page' );

    // Next, detect and handle pagination...
    if ( $query->is_paged ) {

        // Manually determine page query offset (offset + current page (minus one) x posts per page)
        $page_offset = $offset + ( ( $query->query_vars['paged']-1 ) * $ppp );

        // Apply adjust page offset
        $query->set( 'offset', $page_offset );

    }
    else {

        // This is the first page. Set a different number for posts per page
        $query->set( 'posts_per_page', $offset + $ppp );

    }
}

add_filter( 'found_posts', 'adjust_offset_pagination', 1, 2 );
function adjust_offset_pagination( $found_posts, $query ) {

    // Define our offset again...
    $offset = -1;

    // Ensure we're modifying the right query object...
    if ( is_admin() ){
        return;   
    }else if( $query->is_home() && is_main_query() ) {
        // Reduce WordPress's found_posts count by the offset...
        return $found_posts - $offset;
    }
    return $found_posts;
}