Best properly way to destroy a 2 dimensional array?

[Jonathan Villa via Digitalmars-d-learn]

I wrote a little program that given some number it generates a 
list of different combinations (represented by a ubyte array), so 
in the end my function with name GenerateCombinations(int x) 
returns a ubyte[][] (list of arrays of ubytes).

Now the problem is, the quantity of combinations generated are 
pow(2, `x`), thus, giving 20 it returns me a list of 1_048_576 
arrays of ubyte, every array has length of `x` so, in the end 
this function allocates (seeing the Windows Task Manager) 42 MB 
of data after input `20` the first time.

When the list is ready it prints the quantity of combinations 
given (combinations.length), after that I added a foreach(ubyte[] 
a; combinations) calling destroy to every array and then 
destroy(combinations), and before to re-input a new number to 
generate a new combination list I call GC.Collect() and 
GC.minimize().

My question is if I'm doing good calling destroy to those arrays, 
because I don't see any change in the quantity of memory the 
program is using: In the start it uses 1496 KB, after input `20` 
it now have 43000 KB (approx.) and if I input again 20 then it 
sums up to 90000 KB.

In the end I don't see some kind of real memory free. I'm doing 
something wrong?
That's the reason I'm asking for your advice.

PD: All arrays are constructed with the `new` expression.
PD2: Don't ask me why I don't better do an `int count` instead of 
constructing arrays, this little program is going to be part of a 
more huge application and I need the combination list.

Regards. JV