Paging + WP Query

pre_get_posts did the trick (thanks Pieter and cybmeta!

Here’s the solution:

function hwl_home_pagesize( $query ) {
if ( is_post_type_archive( 'motivation' ) ) {
    $query->set( 'posts_per_page', 9 );
    return;
}
if ( is_post_type_archive( 'blog' ) ) {
    $query->set( 'posts_per_page', 14 );
    return;
}
}
add_action( 'pre_get_posts', 'hwl_home_pagesize', 1 );

That’s it!