Bit of an odd question - the Magi are travelling and cannot, for one reason or another, pick up additional supplies and need to ensure that the supplies they have last longer than they would otherwise.
If one of them were to turn their grogs into animals that would eat less than a human, say a mouse, whilst they ate their fill, would those people still be as satisfied with the food they consumed as a Mouse when the transformation ended and they became human again, or would they become hungry again?
Agreed - they would feel full.
You could also change the grogs bodies so they can get sustenance from odd things like soil. They might not like eating dirt but it makes food abundant, and an illusion could make it taste better too.
For an argument, reverse the question. If you turn a camel, which had just drunk the water lasting it for the next days, into a mouse: would it burst and die, because a mouse could never contain that much water?
Keep you saga rules simple and focused, and don't waste time to discuss the digestion of animals in Mythic Europe, unless you absolutely have to!