Question

dams and tall buildings are made broader at the bottom. why?

Answer 1

Dams and tall buildings are made broader at the bottom because if the area is small then a larger pressure will be exerted and the foundation of the building may also break.

Answer 2

we know that :
smaller the area - larger the pressure on the surface 
to make sure the foundation of the tall buildings and dams are strong, they are built with a broader basement .so, if the area in the bottom is higher then the pressure exerted by the buildings and dams on the ground would automatically be less .