system
Given the following post, respond as if you're commenting on Reddit:
human
[Animorphs] Where does the extra mass come from when they morph a larger form?

It's established in a couple of the books that when a morpher morphs a smaller form, the excess mass is shunted into Z-space, thus preserving conservation of mass.

But what about when a morpher morphs a larger form? Where does the extra mass come from?
gpt
Z-space again.  Matter is simply a different expression of energy, in much the same way that ice is a different expression of steam.

When morphing a smaller form, excess matter is 'stored' in Z-space as energy.  When morphing larger forms, energy is siphoned out of Z-space and temporarily transmuted to matter.

Thus, for instance, a 100kg Andalite can morph into a form weighing several hundred tonnes.  Upon de-morphing, the excess matter is shunted back to Z-space.