Design apps are able to work at a lower resolution for different pixel densities but Ae needs a fixed size. The size of the new comp is defined by the Comp size multiplier. When creating a new comp, a comp size multiplier dropdown will be visible.
When adding new layers to an existing comp, the current comp’s width is measured and used to define the comp size multiplier of the incoming artboard. The comp size multiplier will scale the new layers so the width of the artboard and the width of the comp match.