To clarify the difference between 'mass shifting' and 'size shifting', it's been pointed out that a 60+ foot plane can reduce in size, not mass, to a 25 or so foot robot by having parts collapse over, in, or on top of other parts. The dimensions are smaller, but the mass is the same; the smaller form would be far denser that the larger form. Mass shifting would imply and actual change in mass, which is pretty much physically impossible. One might find a way to have Soundwave collapse enough to be the size of a tape deck, but the tape deck would weigh many tons. Of course, movies have ignored this issue. The Hulk comes to mind.