Which has a larger density a large brick or a small brick? why?

Neither

Explanation:

Density is independent of size. Basically, the density of an object isn't determined by it's size.

If two objects are made of the same exact material, they will have the same exact density, regardless of the size.

So, the large brick and the small brick will have the same density, as long as they are made from the same type of brick material.