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?


  1. 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.

  2. 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.

  3. 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.

  • $\begingroup$ Resizing is one of the hardest things to do in a science-based way. Reality simply does not scale up and down, at fundamental levels. Presumably, the atoms and molecules in the organisms are not "resized" -- that would not work out at all with physics or chemistry. Are the cells resized? Then we have to deal with basic issues of volume vs. surface-area at a cellular level. (It's true that some human cells can change size significantly, such as fat and muscle cells.) Are the cells normal-sized, but the number of cells is changed? That last seems the most likely, but perhaps you can clarify. $\endgroup$
    – zeta
    Commented Aug 8, 2015 at 23:13
  • $\begingroup$ @sumelic. It's set in a magical world ;) $\endgroup$
    – durandal
    Commented Dec 9, 2017 at 1:58
  • $\begingroup$ A goose sized duck basically can't even fly, much less a horse sized duck. And tend toward size related health issues. I raised Silver Appleyards for a while - lovely sweet ducks, definitely couldn't get off the ground, most of them ended up with heart problems and early demise. $\endgroup$ Commented Jan 27, 2022 at 21:44

3 Answers 3


A horse-sized duck would almost certainly just collapse,. Being big has a biological cost -- otherwise, all animals would be the size of the Burj Khalifa. You need to have special infrastructure to deal with all that bulk, and just increasing the size of your existing infrastructure isn't enough.

Birds are already stretching the limits of their infrastructure, since they need to be as light as possible to fly.

The effects of reducing the size of a horse would depend on the process involved, but you can have a discussion of some sort, at least. It would probably be able to survive for a while, since it can still graze, though it would be too slow to outrun any of its predators, and it will probably gain new ones.

  • $\begingroup$ I sometimes wonder how the largest dinosaurs managed to survive. You'd think that they would suffer all sorts of problems, especially from the blood pressure needed to keep their circulation going. For example, "Sauroposeidon ... probably grew to 18.5 metres tall ." google.co.uk/?gws_rd=ssl#q=tallest+dinosaur $\endgroup$ Commented Aug 8, 2015 at 16:27
  • 3
    $\begingroup$ A duck sized horse might do better (ignoring predators) given Ancient Warming Shrunk Horses to Housecat Size - of course, that is not a horse as we know it and might have had other adaptations suitable to the smaller size & warmer climate for which it may have had thousands of years to gradually adapt. $\endgroup$ Commented Aug 9, 2015 at 6:48

One of the ancestors of the horse (Eohippus) was about twice the size of a Fox Terrier, around 30 pounds.


While a really big duck (Muscovy) weighs only about 15 pounds. A horse the size of a duck would literally be a sitting duck, but ignoring that, horses are relatively inefficient eaters and need the bulk to get enough nutrition.


No matter whether you scale a creature up or down, the square-cube law will be your nemesis. It's hard to work around it without changing the creature's biology in extreme ways.

The most physically-sensible way that this kind of magic could work would be changing the creature's genetics somehow--basically taking apart and reassembling the creature at the cellular level, turning it into a differently-sized subspecies. To make it look identical while being larger or smaller would require some nonsense about "changing the spacing between atoms," which would kill the creature in a dozen different ways. Explosive ways.

Within a reasonable size range, resizing by spontaneous genetic alteration could work. Some adaptations would be needed: a shrunken horse would become stocky like a pony, while an enlarged duck would be flightless and ostrichlike. If you assume these kinds of alterations, I'd say the horse-sized duck would have an easier time surviving--even though it wouldn't quite be a duck anymore.


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