Why does sortedDictionary need so much overhead?

Question

long b = GC.GetTotalMemory(true);
SortedDictionary<int, int> sd = new SortedDictionary<int, int>();
for (int i = 0; i < 10000; i++)
{
    sd.Add(i, i+1);
}
long a = GC.GetTotalMemory(true);
Console.WriteLine((a - b));            
int reference = sd[10];    

output (32 bit):

280108

output (64 bit):

480248

Storing the ints alone (in an array) would require about 80000.