In a pre-industrial world where magic is thriving, it is possible for a mage to alter the size of a creature (including other humans).
Thus a duck could be made horse-sized. It would have all the normal features of the original duck but its mean density would remain the same. Thus it would be very unlikely to be able to fly. Similarly a horse could be made duck-sized. It too might have problems, for instance being chased by a cat.
There are magical limits to the transformations. Creatures can be no smaller than 1/100 of their natural size and no bigger than 100 times their natural size.
However I am interested in the physical limitations. Most wizards have a purpose for the transformation, they don't usually wish to kill the animal by making it unfeasibly large or small.
Animals are physically suited to their natural sizes - their musculature and skeleton etc. are 'just right'
What level of transformation is likely to produce viable creatures? I am primarily interested in survival. The animal needs to be able to survive long-term in its altered state in terms of avoiding danger and acquiring water and food etc. How will the change affect a creature's mobility?
What other implications may limit what mages can sensibly do in this respect?
The tag 'science-based' relates purely to the physical effects of changing size whilst maintaining the same overall density and keeping the same shape. The magic has its own rationale which can be considered as separate and is not relevant to the question.
When a brain is changed in size, the intelligence and psychology of the original creature remain unchanged. A duck still thinks it's a duck and presumably experiences the world as smaller and gravity as stronger.
Successive transformations, e.g. smaller and smaller are not possible. You must restore a creature back to its original size before transforming it again.
The title was inspired by the Horse-Sized Duck meme.