Very large collection in .Net causes out-of-memory exception

I am testing how big a collection could be in .Net. Technically, any collection object could grows to the size of the physical memory.

Then I tested the following code in a sever, which has 16GB memory, running Windows 2003 server and Visual Studio 2008. I tested both F# and C# code, and looked at the Task Manager while running. I can see that after about growing 2GB memory, the program crashed with out-of-memory exception. I did set the target platform to x64 in the property page.

open System.Collections.Generic

let d = new Dictionary<int, int>()

for i=1 to 1000000000 do
    d.Add(i,i)

I did a same test to the C5 collection library. The result is that the dictionary in C5 could use up the whole memory. The code uses C5:

let d = C5.HashDictionary<int, int> ()
for i=1 to 1000000000 do
    d.Add(i,i)

Anyone knows why?