Final answer:
Concrete columns are often thickened at the top to improve structural stability, counteract lateral forces, and sometimes to create an optical illusion of strength.
Step-by-step explanation:
Concrete columns are typically thickened at the top to counteract the lateral thrust of the vaulting and to ensure the stability of the building. This practice also compensates for the dynamism and organic feel that structures may exhibit, as with the slight bulge in the entasis of Doric columns. The thickness of such architectural elements can be crucial, similar to how the thickness of a dam increases with depth to balance the increasing force due to the increasing pressure. In architectural history, it has been noted that columns thicker at the top can also provide an optical illusion of strength and solidarity.