As we know liquid pressure increases with depth. So, to prevent the dams from collapsing due to the enormous liquid pressure, they are made wider at the base.
In the diagram above we can see a dam. The dam is thicker at the bottom while thinner at the top. If the dam were thinner at the bottom the increased pressure at the bottom of the reservoir would push the dam away and destroy everything. This is why the bottom of the dam is made thicker so that it could withstand strong water pressure which increases by the increase in the depth.