Grants come under different powers. Councils have a number of specific powers. They can also use section 137 of the LGA 1972 to spend on anything that gives commensurate benefit to citizens. But I don't know of anything that says the size of a grant requires different rules. A local decision could be made on that basis, but I would expect there to be some policy at every level. It's not mandatory, but it is certainly good practice.