Here's why:
* Crystallization: As magma cools, minerals start to crystallize. The slower the cooling rate, the more time the minerals have to grow into larger crystals.
* Depth of Formation: Coarse-grained rocks typically form deep underground, where the heat from the Earth's interior keeps the magma hot for a longer period.
* Insulation: The surrounding rock acts as an insulator, slowing down the rate of heat loss from the magma.
In contrast, fine-grained igneous rocks, like basalt, cool rapidly, resulting in smaller crystals.
Let me know if you'd like more information about igneous rocks!