Filter/hook to add class to h1 on single-product.php

I’m looking at my Single Product title template:

the_title( '<h1 class="product_title entry-title">', '</h1>' );
And I want to add a class (‘small’) to the title without modifying the template (to avoid problems with future updates).

I’d like to plop in a filter but don’t know how to do this. I suspect it’s something like this:

  add_filter( 'woocommerce_product_loop_title_classes', 'custom_woocommerce_product_title_classes' );
    
    function custom_woocommerce_product_title_classes( $class ) {
        return $class . ' small'; 
    }

Which seems to work for the shop page, but not the single product page.