For a given angle of view, the amount of light incident on the sensor depends solely on the diameter of the aperture (the absolute diameter, not the f number). You can see this visually in the comment I made here: https://news.ycombinator.com/item?id=33426540
If you really want to think of it in terms of a combination of sensor size and f number, you can do so. But it's easier just to look at the size of the hole the light is going through – which not surprisingly, determines how much light ends up being collected, once you fix the angle of view.
>If I was shooting at night, why would I use 400 iso?
Because films with higher ISOs are unacceptably grainy for most uses.