I am not sure why the character "小" (small) is used here. Is there any difference between "小炒牛肉" and "炒牛肉"?
To understand the meaning of 小炒, you need some more background information.
Chinese factories or companies normally employ chefs to cook for employees, and chefs need to cook with super big pot or stew. This kind of dish is called 大锅菜, literally it means big pot dish. Compared with 大锅菜, there are also dished cooked with small pot and more work. This kind of dishes are called 小炒, literally it means small stir-fry. So slowly, 小炒 is used to describe dishes that need special care and more work. Compared with 炒牛肉, 小炒牛肉 needs more care and work.