The more water vapor a magma contains, the more fluid it is. Now, it stands to reason that the easier a magma can flow, the easier it is for things such as atoms to move through the liquid. For example, it's easier for you to swim through water than it is through cool jello. Anyway, the easier it is for atoms to move through a magma to places where crystals are forming, the larger these crystals can become. And the longer the magma stays liquid, the larger the crystals can grow.
Porphyritic textures form primarily because of two rates of cooling.
Most often, this happens when a magma begins to crystallize at depth, then moves upward to the surface before it is completely solidified. At the surface, it cools more rapidly, resulting in the formation of a porphyritic texture where the early formed larger crystals are now surrounded by a finely crystalline, more rapidly cooled groundmass. All the granite in the earth's crust can be accounted for, volume-wise, by differentiation of a magma of mafic composition. B. Evidence for assimilation of country rock by an igneous intrusion includes things like fuzzy boundaries between the intrusion and the country rock, and chemical changes in the igneous rocks near the edges of the pluton. C. Granite in subduction zones often originates by partial melting of mafic ocean crust and sediment, which in turn generates a felsic magma.