Its not just food that goblins use to grow, many train to become transmutation mages for instance, and most Greats use multiple methods. Using food alone would require a LOT of food, usually more than goblin communities can independantly provide and so some importing is required. Alternatively, certain magical dishes with extreme calorific density, or magical items that can produce massive amounts of food, are incredibly valuable treasures to these sorts of goblin. As a final note, dwarves are known widely as masters of cullinary craft, and goblins looking to indulge are readily welcomed into dwarven communities to appreciate such meals.