You have the thumbnail setting in Virtuemart configuration. Make sure its 250px at least
You also have margins in Customizer .. Go to the cuztomiser and see if you can adjust things that is here.
This will also affect other areas of the shop so it might be better that you go to the files of this in the html folder in your template. /web/salubrious3/templates/yoo_nano3/html/com_virtuemart/themes/grids/categories
Check these files. .. I susually add <p>test</p> somewhere to check where Im at in those files. Sometimes it can be hard to se this.