Since you're dealing with some hand-waving anyway (Where does all the mass go?), some or all of these could be used. It strikes me that the shape-shifter might treat non-self transformations as memories of dreams. As such, a non-self transformation, when dealing with memories or processing can be looked at through the lens of lucid dreaming.
When the animal in question is of a much lower physical capacity than the shifter, that's ok, because the shifter knows it to be a dream. The shifter knows the rules of the game still apply (like hungry larger creatures are still hungry and in their dream, can have a snack of the shift's dream self), but the shifter may in fact walk an uncanny edge by behaving more like a sentient shifter than a bird, for example.
Looking at it through the lens of a wider fantasy setting, this means that attempting to impersonate a creature with greater faculties may give the impression of a mentally challenged member of that species, as the native members would see the behavior and/or mind of the shifter to be lacking something that's inherently them.
Biology-wise, our active regions are quite voluminous when compared to the size of a crow's brain, yet the crow can use tools, have fun, experience elder family bonds, and solve puzzles, so it is not too far fetched that some capacity can remain with a resized brain. For extreme resizes (beyond an order of magnitude, say, to a cockroach from human), you might look at "heat" penalties for advanced thought, carrying a risk that some crucial link to the shifters' self can get melted by too high a current, after which they lose themselves and become what they've transformed into, allowing for new identities to be born. This also provides an opportunity to develop from a story standpoint as a shifter who is not as well fortified against this effect and even reducing their brain mass by 2% gives them headaches. Alternately, paragon shifters can transform into dragonflies and actually reach a destination, as long as they don't try to think about philosophy in the background. Maybe this is where Zen as a concept comes from in this world?