Eiffel Bridge Design

In bridge design normally base supports are strong,fixed/rigid. Why did he choose thicker arch sections at middle above mid-river and narrow sections at abutment supports? If he chose a simply supported condition, then for what structural advantage?

EDIT1:

Could it be some combination of arch and beam design?

Between arch abutments it spans 5 pillar span lengths.Treating main structure as a long span Beam bridge simply supported at ends bending moment at mid-span is maximum so midsection should be designed deeply trussed as done here.

However does it not compromise benefits of pure membrane arch action ( like St. Louis Gateway arch, Missouri design as a catenary) creating stress concentration by lower section property utilization?

Eiffel Rail-Road Bridge Portugal

“In 1876 they designed the 1,158-foot steel railroad Ponte Maria Pia (Maria Pia Bridge) over the Douro River in Oporto, Portugal”.