As per the title,  what is the line to define WG?  Especially when there is Expansion and stuffing. I mean assuming expansion characters would rapidly grow when is it not "WG"?

That is a fair question. Personally I tend to find that the distinction between expansion and weight gain tends to be roundness and softness. Fat tends to sag and roll, where expanded assets tend to be a bit more perfectly shaped. I think a good rule of thumb would be to stick with limited expansion, for example, belly inflation, where the belly gets round and tight, as opposed to soft. I hope that makes it a little more clear :)