Why are fullerene cages smaller than C60 generally less stable?

Fullerene cages smaller than $C_{60}$, such as $C_{28}$, $C_{36}$, and $C_{50}$, are generally considered metastable because they possess higher levels of strain compared to the exceptionally stable $C_{60}$ structure.1

In chemistry, the stability of a ring or cage structure is closely linked to the deviation of its bond angles from ideal values; greater deviation results in higher strain and lower stability. While $C_{60}$ forms a highly stable, closed spheroidal cage—a truncated icosahedron consisting of 12 pentagons and 20 hexagons—smaller cages face geometric constraints that increase this internal strain. These smaller species are often characterized as metastable, meaning they are less chemically stable than the $C_{60}$ cluster and are typically observed only under specific experimental conditions rather than as naturally abundant, highly stable forms.21