If you think of the inside of a female fish as being a fixed size, she only has so much room for eggs. The larger each individual egg is, the fewer she can make at a time. There are other, less obvious things affecting egg size. Hatching time is one of these. The time it takes an egg to hatch is dependent on the size of the egg.
Longer eggs take longer to hatch and develop than smaller eggs.
This can be doubly important with cichlids because the parents take care of the eggs and fry. This means that any extra time a larger egg takes to hatch and develop is extra time the parent or parents must spend watching over them. It delays the time when the parents can spawn again. Its even more important for a mouthbrooder that can't feed while it has eggs in its mouth. The larger the egg, the longer it takes to hatch, and the longer the parent goes without food.