The igneous texture characterized by two distinctively different crystal sizes is called porphyritic.

Here's a breakdown:

* Phenocrysts: The larger crystals in a porphyritic texture. They form early in the cooling process, when the magma is still relatively deep underground.

* Groundmass: The smaller crystals or glassy matrix surrounding the phenocrysts. They form during the final, rapid cooling of the magma as it erupts onto the surface or solidifies near the surface.

Porphyritic textures provide information about the cooling history of the igneous rock. They indicate that the magma experienced two distinct stages of cooling:

1. Slow cooling: This allows the large crystals (phenocrysts) to form.

2. Rapid cooling: This causes the smaller crystals (groundmass) to form.

Let me know if you'd like to know more about igneous textures or other aspects of geology!