Why is Li smaller than Li?

It seems there's a typo in your question, as it compares lithium (Li) to itself. If you meant to compare lithium (Li) with another element, such as sodium (Na), lithium is smaller due to its higher effective nuclear charge and fewer electron shells, resulting in a stronger attraction between the nucleus and its electrons. This leads to a smaller atomic radius compared to larger elements with more electron shells, which experience greater electron shielding.