Memory size grows exorbitantly with dynamic 2D memory allocation

I have the following piece of sample code allocating 1D array:

#define C 3
int main() {
    int *a;
    long long N = 1000000000, i;
    a = (int*)malloc(sizeof(int) * N * C);
    for (i = 0; i < N * C; i++)
        a[i] = i / 2;
    printf("%d\n", a[N*C - 1]);
    return 0;
}

The above code takes exactly 12 GB of data in memory.
Note that sizeof(int) == 4 and sizeof(int*) == 8.

Now, if I implement a dynamic 2D array using the following code:

#define C 3
int main() {
    int **a;
    long long N = 1000000000, i;
    a = (int**)malloc(sizeof(int*) * N);
    for (i = 0; i < N; i++)
       a[i] = (int*)malloc(sizeof(int) * C);
    for (i = 0; i < N; i++)
       for (j = 0; j < C; j++)
           a[i][j] = i;
    printf("%d\n", a[N-1][C-1]);
    return 0;
}

The above code strangely takes around 38 GB of memory (whilst it should have taken 12GB + 8GB (for array of pointers) = 20 GB.

Strangely enough, in the second sample code, if I increase the value of C to 4, 5, 6, the memory consumed is exactly the same (38 GB), while for C=7 and C=8 the memory consumed is 54 GB, and for C=16 the memory consumed is 86 GB. This doesn't fit any math I could figure out. Can anyone help me out with this?