I suppose that makes sense. and Revan4559 all of your examples are correct. But I think(THINK) that we are referring to Magnus physically not in the void, and not after he became a Demon Prince of Tzeentch. As far as I know after he fought the Space Wolves and became a deamon prince, then he constantly is changing his appearence. In Battle for the Fang. He does go back to the appearance his son's remembered him as originally. I forget why he does this (to inspire them, or for nostalgia?.) But it's true, he can change sizes, or at least others perceive him that way. IT IS also true that he while in the Void can change his size.
But in real space, before his turn to Chaos...thats the part that is I think debatable. Though DeathJester makes a pretty valid argument I find difficult to beat.