Sometimes rock has both large and small mineral crystals. how can you explain this?

Well, different rocks may have gone through different geologic processes. For example, rocks that have undergone lots and lots of heat and pressure may have different size grains than rocks who have undergone lesser amounts of heat and pressure. 

He reason why minerals have different sized crystals is for many reasons. One because crystals grow bigger over time and two some minerals might not be that old. And three some just grow faster than other due to the properties of there surroundings