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I found it...

editing [root]\themes\[my_theme]\templates\catalog\listing\category.tpl

<img class="img-fluid" src="{$link->getCatImageLink($subcategory.link_rewrite, $subcategory.id_image, 'category_default')}" alt="{$subcategory.name|escape:'html':'UTF-8'}"/>

change into:

<img class="img-fluid" src="{$link->getCatImageLink($subcategory.link_rewrite, $subcategory.id_image, 'small_default')}" alt="{$subcategory.name|escape:'html':'UTF-8'}"/>