Answer:
Metamorphic rocks are denser than igneous rocks, because of the way they are formed. To explain:
Metamorphic rocks are formed when all types of rocks are compressed under high temperature and pressure. Having been compressed under high pressure, they become denser.
Igneous rocks, however are formed when magma freezes. It could just freeze when it reaches a slightly colder area and is not affected by the pressure at all. Or if it was, it wouldn't be igneous anymore.
Hope this helps