i like to think that this apparent breach of the conservation of mass has been taken care of with some advanced alien technology. or the physical properties of z-space, or some other, previously unknown dimension.
or, as a last resort, i fall back on my sunday school animorphs answer:
it was all the ellimist.
otherwise, the animorphs would have been filleted three ways from sunday before tobias would ever have had to worry about that pesky two-hour time limit.