Well the first thing is that it feels very intuitive that changing a man into a bear is an act of transformation, whereas adding more water to a pool or making a rock grow to 4 times its size feels more like an act of creation.
Yes, in this instance, it makes perfect sense to me that you could transform a small piece of rock into a narwhal tusk with Muto. But to make that rock into a much larger piece of rock feels very different.
What is your solution to the fact that the Muto Aquam spells to cause lakes to flood, etc., are conspicuously missing from ArsM5? Is your approach that you can use Muto if you want but it just isn't included in the base book? Or is it that water is an exception to the general rule about Muto being able to cause an increase in a mass of a particular substance, such as rock? (I feel like that final one may be the thought process behind RAW, but I'm not sure I can justify it very well).