The bulk discount is only applied for products in specific and I don't find it visible the option to create a bulk discount for a whole category.
Example:
1 and 2 pdct 15%
3 or more pdcts 30%
In this moment if you want to offer a collection discount is not possible to give a round discout because if the customer buys 3 different products instead of applying 30% on the purchase as it could be offered, he gets a 15% for each product.