Several paranormal and supernatural series have “shifters” now, humans that can turn into other creatures, animals or otherwise. Unlike weres – werewolves, werecats and werecoyotes that can only shift into one form and often are affected by moon phases – shifters can take on multiple forms.
My question, however is this: conservation of mass. To make this example simple, the modern Avengers Hulk (“Son, you gotta condition”) doesn’t make sense whereas the old Bill Bixby and Lou Ferrigno tv “Hulk” did. Why? Because you can imagine something getting a little bigger, but where does the mass of something ten times larger come from? How is all that energy stored, and where does it go when it isn’t in use?
Both “True Blood” and the Twilight movies make use of shifters. In Twilight, the wolves appear four times larger than their human counterparts. In “True Blood,” Sam can shift into a fly! Where does 160 pounds (wringing wet) disappear to unless that’s going to be a HUGE fly? Just as incredible is Sam’s ability to find jeans that fit him perfectly every time he shifts back to human form no matter where he is, but I digress.
So, does it strain credibility when a character shifts shape into a creature too large or too small to be believable? Yes, its magic or mysticism or whatever, but does it help suspend disbelief when the shifting is done into something of approximately the same size and perhaps relative shape?
ThinkingSkull.com 
For the living Were’s and people like the hulk, the mass exchange is due to the accelerated cell reproduction that goes along with an “healing factor” For generations, Weres have been known for their ability to heal and even regenerate. if you can force the cell structure to go into overdrive, to heal, you can also use the same ability to grow as well. The mass can only be pushed to a given multiplier of your base self, usually 1.5 to 2x the size, before the ell structure reaches it’s limit. In the transformation, you have to have some sort of healing, because your body would tear itself apart otherwise. By the application of healing, and the simple size figures, it is plausible to have the mass conversion you describe.
As for someone like the Hulk, who has been known to grow bigger as he gets madder, or just the sheer size, the same theory applies, just with a psychological factor added in. We have seen time and again, our emotions can force changes in our physical selves. The Hulk is just the extreme version.
As for the Vamps, I have not worked out the shrinking part, unless you subscribe to the same mass theory. You get smaller, you get denser. We are made up of a lot of air and water, so if you could shrink you cells and make them go closer to take out the negative space, the shrinking then becomes an idea as well.
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I liked how in Ang Lee’s “Hulk” movie, when he calmed down, it was like fluid was draining out of him and evaporating, as if the gamma radiation somehow catalyzed the air and absorbed the humidity to increase his bulk and maintain it.
All cool no-name, non-bronze, no-prize worth explanations aside, does significant size increases or decreases strain the suspension of disbelief of an audience or reader in enjoying a story?
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