When using Muto to change one material to another, it is possible to end up with a different amount of material than you started with, depending on the base individuals used in the spell. An example of this can be found in the Muto Terram guidelines on page 154:
The amount of material you can change is limited by the guidelines for each end. Thus, you can turn a cubic pace of stone into a cubic inch of gemstone with a base Individual.
However, there are two MuAu cantations in Apprentices for which this shrinking is stated to be required, Mimic the Christ's Miracle and An Evening's Illumination. These spells must transform a barrelful of water in order produce a cupful or bucketful of a processed liquid:
Changing water into a processed liquid reduces its size, and it takes a barrelful of water to produce a cup-sized amount of wine.
This spells changes water into lamp oil. As earlier, a barrelful amount of water will produce a bucketful of oil.
Some users have extrapolated this shrinking requirement to apply to all Muto Aquam spells, or even all Muto spells in general, so long as a change in base individual is involved. Personally, I think this interpretation is incorrect because it would alter the effect of many spells, and contradicts some errata, but I will go over each interpretation that I have seen.
Applied to All Muto Spells
The spells in Apprentices are the only Muto spells explicitly stated to behave this way, and it would cause a number of other spells to behave very differently than a casual reading of their effect would imply. For example, Rope of Bronze has as targets a plant 1 pace in each direction (27 cubic feet), and a base metal (1 cubic foot). Were we to extrapolate from the spells in Appprentices, casting Rope of Bronze would shrink the targeted rope to a third of its length.
Extrapolating this requirement would also contradict with the errata for Earth That Breaks No More. The pre-errata description of the spell was as follows:
Turns a volume of packed dirt up to one hundred cubic paces into stone.
As a T: Part spell with +1 Size, it was valid to target one hundred cubic paces of dirt (10 times base target 10 cubic paces), but not stone (10 times base target 1 cubic pace is only 10 cubic paces). As per the rule on page 154, this would have only produced 10 cubic paces of stone from 100 cubic paces of dirt. Earth that Breaks No More eventually received the following errata:
Change the description to read "Turns a volume of packed dirt of up to ten cubic paces into stone".
Now the spell's maximum targetable amount of packed dirt no longer exceeds the ten cubic paces on the stone end. Under the interpretation extrapolated from Apprentices, the post-errata spell would only produce up to a single cubic pace of stone, and the +1 to the spell's magnitude for increased size would have no impact and thus be erroneous.
Applied to All Muto Aquam
There are also a few issues with applying this to the entirety of Muto Aquam. First of all, Bag of Teeth can transform an entire base individual of water into a single animal tooth. This is smaller than the base target for animal, which would contradict the proportionality required by Mimic the Christ's Miracle.
The mandatory proportionality would also cause minor problems with spells such as Lungs of the Fish: each breath of water would create more air than one would expect.
Applied to Only Muto Aquam (fluid->fluid) transformations
I can only find one existing spell that would still be affected by this very limited extrapolation, the ritual Strange Tricks Using Canal Contents from Transforming Mythic Europe. It would, however, prohibit the creation of a water-to-wine spell that does not cause the glass of wine to expand into a barrelful of water after it has drunk.
Applied to Only The Spells In Apprentices
Personally, I'm in favor of treating the proportionality requirements of Mimic the Christ's Miracle and An Evening's Illumination as quirks of those particular cantations rather than extrapolating from them as a rule for other spells, either due to a flaw or deliberate design. On the one hand, a cantation learned through repeated casting might not be as thoughtfully designed as a spell created in the lab. On the other hand, making wine that causes the drinker to explode with water fits in perfectly well with other cantations like Curse of the Baby's Bladder, especially if the storyguide interprets the effects of suddenly having the glass of wine you drunk suddenly become a barrel of water as more humiliating than life-threatening.