How does heart size relate to the size of an animal?
Generally the larger the animal, the larger the heart.
Large animals have large chest cavities, capable of supporting large hearts. And they need them too - big animals like whales have a lot of cells that need blood delivered to them. Humans, being not too big, not too small, have a moderate heart size - about the size of a grown man's fist. We don't need big hearts because we don't have big bodies; having a heart that reflects this allows our chest to support other organs like the lungs.
As an interesting side note, the larger the animal, the slower its heart beats, and the smaller the animal, the faster its heart beats. A blue whale's heart beat, for example, hovers around 9-10 beats per minute while the Etruscan tree shew, an animal so small it can fit in the palm of your hand, is known to have a heart rate of over 1000 beats per minute.