AJAX Call is Only Returning 100 Results from Query

I have this AJAX call to fill a dropdown:

function get_select_companies()
{
    var t_id = $('select#select-product_types').val();
    if( t_id == -1 )
    {
       $('#ajax-loader').hide();
       $('#select-manufacturers').hide();
    }
    else
    {
    $('#ajax-loader').show();
        $('#select-manufacturers').hide();
        $('#select-manufacturers option').remove();

    jQuery.ajax(
        {
            url:     '/wp-admin/admin-ajax.php',
            type:    'POST', 
        data:    (
            { 
            action:         'px_get_select_companies', 
            type_id:        t_id 
            }),
                dataType: 'JSON',
        success: function(data)
                {
            $('#select-manufacturers').append(data);
            $('#ajax-loader').hide();
            $('#select-manufacturers').show();       
        }
        });
    }
}

I then have this php function on my functions.php file

function px_get_select_companies()
{
$args = array(
    'post_type'     => 'products',
    'posts_per_page' => -1,
    'tax_query' => array(
        'relation' => 'AND',
        array(
            'taxonomy' => 'product_types',
            'field' => 'id',
            'terms' => array( $_POST['type_id'] )
        ),
    )
);

$query = new WP_Query( $args );

while( $query->have_posts() ) : $query->the_post();
    if( $query->post->ID != 1 ) :
        $terms[] = get_the_terms( $query->post->ID, 'manufacturers' );
    endif;
endwhile;

foreach( $terms as $term ) :
    foreach( $term as $ID ) :
        $term_IDs[] = $ID->term_id;
    endforeach;
endforeach;

$term_IDs = array_unique( $term_IDs );

$output = '<option value="0" selected="selected">--- Επέλεξε Εταιρεία ---</option>';

foreach( $term_IDs as $ID ) :
    $term = get_term_by( 'id', $ID, 'manufacturers' );
    $output .= '<option value="' . $term->term_id . '">' . $term->name . '</option>';
endforeach;

wp_reset_postdata();

$output=json_encode($output);

if(is_array($output)){
    print_r($output);   
}
else
{
    echo $output;
}
die;
}

The problem is that I get an endless spinner as it’s not returning any results. If i change posts_per_page to 100 or less, it works as expected. Anyone have an idea as to why it’s doing this?

Thanks