Why the assembly Mono.Cecil wrote is smaller than original?

Question

Here is my test code AssemblyDefinition assembly = AssemblyDefinition.ReadAssembly(@"E:\vd\fishrun_apk\assets\bin\Data\Managed\System.dll");

assembly.Write(@"E:\vd\fishrun_apk\assets\bin\Data\Managed\System.dll.new");

System.dll.new and System.dll have different size? Could somebody tell me why?