At the time the Playstation pushed more polygons per frame. I remember this debate because Nintendo said that they allowed for larger seamless polygons and said that it produced a better image on the screen.
So a system can have double the resolution of another and games can even choose to render at different resolutions depending on how intensive it is. Some games will choose lower resolution but more polygons and some might choose higher resolution with fewer polygons, among other choices. There are even modern cases where console games will change the resolution they run at to keep framerates solid https://www.halowaypoint.com/en-us/forums/6e35355aecdf4fd0ac...