Clear and Unset doesn't free memory

Example: Try this example only if you have > 4G ram

d = 2000; 
f[x_, y_, d_] = 

     Sum[Subscript[a, i, j]*x^i*y^j, 
       {i, 0, d - 1}, {j, 0, d - i}];

Then

Clear[f]

or

f[x_, y_, d_] =.

You will see that the memory usage is not reduced after calling Clear or using Unset.

Question: How to free the memory when I don't need a function anymore which is already evaluated?

Edit: I look at the memory usage in taskmgr under Windows OS. As the answers report that the memory is indeed reduce by using MemoryInUse to check it.