Here's why:

* Electrons are in constant motion: They occupy a fuzzy region of space called an electron cloud, which is much larger than the nucleus.

* The nucleus is tiny: While containing most of the atom's mass (protons and neutrons), the nucleus is incredibly small compared to the overall atomic size.

Think of it like this: Imagine a stadium with a tiny marble in the center. The marble represents the nucleus, and the stadium represents the electron cloud. Even though the marble is very dense, the stadium (electron cloud) is what defines the overall size of the atom.