Problem using BSTR in ActiveX-function

If I have a function inside ActiveX-control:
STDMETHOD(put_Text)(/*[in]*/ BSTR newVal);


and I call it from my application with string length more than 530000, then the program crashes (error dialog comes).
Why? Funktion works if the length of the string is less than 500000. Is there some limit for strings (BSTR) when using ActiveX-functions?

Here is my example-code (m_Dase is my ActiveX-control object):
CString ss = "12345678901234567890123456789012";
for(int i=0; i<14; i++)
{
ss = ss + ss;
}
// SetText (=put_Text) is a ActiveX-function
m_Dase.SetText(ss); // CRASHES HERE