You are referring to the size of crystals in different types of basalt, not the number of crystals. Here's why pillow basalt crystals are typically smaller than those in other types of basalt:

1. Rapid Cooling: Pillow basalt forms when lava erupts underwater. This rapid cooling of the lava is key. The cold water quenches the lava quickly, preventing large crystals from forming.

2. Fine-Grained Texture: The rapid cooling leads to a fine-grained texture called aphanitic (meaning you can't see the crystals with the naked eye). This is why pillow basalt often has a glassy appearance.

3. Crystal Growth Time: Crystals grow over time as lava cools and solidifies. The faster the cooling, the less time there is for crystals to grow.

4. Cooling Rate Variation: Basalt that cools more slowly, like in thick flows or on land, allows for larger crystals to form. These basalts are often called phaneritic, meaning you can see the crystals with the naked eye.

In summary:

* Pillow basalt: Rapid cooling, fine-grained texture, smaller crystals.

* Other basalt types: Slower cooling, coarser-grained texture, larger crystals.

Let me know if you have any more questions!