AES Encryption 256 bit

Hello,

I am encrypting data using the below AES code:

Public AesIV As String = "1233123456789012"
Public AesKey As String = "1233123456789012"

Private Function aesEncrypt(strData As String) As String
' AesCryptoServiceProvider
Dim aes As New AesCryptoServiceProvider()
aes.BlockSize = 128
aes.KeySize = 128
aes.IV = Encoding.UTF8.GetBytes(AesIV)
aes.Key = Encoding.UTF8.GetBytes(AesKey)
aes.Mode = CipherMode.CBC
aes.Padding = PaddingMode.PKCS7

' Convert string to byte array
Dim src As Byte() = Encoding.Unicode.GetBytes(strData)

' encryption
Using enc As ICryptoTransform = aes.CreateEncryptor()
Dim dest As Byte() = enc.TransformFinalBlock(src, 0, src.Length)

'encode encrypted bytes to URL safe string and return

Return HttpServerUtility.UrlTokenEncode(dest)

End Using
End Function

As you can see I currently set the aes.KeySize to 128 bit andURLTokenEncode the return data.

My question is this: how come when I change the aes.KeySize to 256 (and the AesKey to 32 character length) the returned encrypted string is exactly the same as it is for 128 bit? It doesnt look like theaes.KeySize code is doing anything at all. I just need some clarificiation if possible!

Thanks in advance