Damn those are some AWESOME screenshots.
That basically 99.9% confirms that the Winged Muto at least, will have to change sizes as the film goes on or there is more than one of them.
Because the shots of him diving into the water, and straining against the wiring he is WAY smaller than in this shot now where is grappling with Godzilla.
Unless they just screwed up the scaling in the shots really bad. I mean, scaling issues are impossible to 100% avoid, but I don't think Gareth would have it be THAT bad.
So yeah: #morethan2Mutosorsizechanging/growingMutosconfirmed! :-)