Here's why:

* Cations are positively charged ions formed when an atom loses one or more electrons.

* Helium is a noble gas with a full electron shell, making it extremely stable and unlikely to lose electrons.

* Helium's atomic radius is already extremely small due to its two protons tightly holding onto its two electrons.

While Helium doesn't readily form cations, theoretically, if it did lose its electrons, the resulting cation would be the smallest due to the absence of any electron cloud.