BinarySerialization size not consist?!

Hello all,

I have met something pretty odd.
I am running a serialization on an object and not always the size of
the output stream is the same.
I even created a test that I am running the serialization in a loop,
and each time I am running I have different results sizes in some
point in the loop.
This happens when I am using a object that was filled in the server
side, and I get them thru WCF - working with NHibernate DAL, but the
code is running on the client.
Here is some of the code I am using for the loop:
AAAA aaa = new AAAA();
aaa.NewMethod();
AssistanceFileEntityBankAccount
m_AssistanceFileEntityBankAccount=aaa.AFBE;

long size = 0;
MemoryStream ms = new MemoryStream();
BinaryFormatter bf = new BinaryFormatter();

var entity = new Student();
entity.Num = 11123456;
entity.AccountClosingDate =
m_AssistanceFileEntityBankAccount.AccountClosingDate;

//without this line, everything works!
entity.AssistanceFileEntity =
m_AssistanceFileEntityBankAccount.AssistanceFileEntity;
for (int i = 0; i < 100; i++)
            {

                bf.Serialize(ms, entity);
                ms.Seek(0, SeekOrigin.Begin);
                Debug.WriteLine(ms.ToArray().ToString());
                if (i > 0 && size != ms.Length)
                {
                    Debugger.Break();
                }
                else
                {
                    size = ms.Length;
                }
            }

Each time I stop in a different I, the values are not equal, Is this
make sense??!
Any Ideas will be appreciated.

Thanks,
Efy