A heart, when referring to something very small, could represent several things depending on the context:

Literal Meaning: A tiny representation or symbol of a heart, like a miniature heart-shaped object or a small drawing.

Metaphorical Meaning: It might symbolize a small amount of love, compassion, or emotional significance. People often use the heart as a symbol for emotions, so a "very very very small heart" could suggest something that has very little emotional impact.

Biological Context: If you are referring to an actual heart, there are tiny hearts in nature, such as the hearts of small animals, insects, or even the heart of a developing embryo in the very early stages of life.