msdn.microsoft.com/en-us/library/eahchzkf.aspx
Marshal.SizeOf returns the size after the type has been marshaled, whereas sizeof returns the size as it has been allocated by the common language runtime, including any padding.
и:
msdn.microsoft.com/en-us/library/vstudio/5s4920fa(...
The size of the specified type in unmanaged code.
т.е. sizeof - размер типов в CLR, а Marshal.SizeOf - размер типа в unmanaged коде.
И по первой ссылке есть пометка по этому поводу:
Although you can use the Marshal.SizeOf method, the value returned by this method
is not always the same as the value returned by sizeof.
