tagged protein size - (Jan/30/2013 )

Hi
thanks again for response, ok I got the whole point but in many cases I notice that my protein size is bigger than it should be like in X protein size around 20 kd with His which I supposed it should be 20 either so my protin appeared around 40 kd I checked the frame and there nothing with it, that is what I could not explaine it?

many thanks