thats because the size of the nested isn’t changed automatically when the parent comp is.
You could fix it by multiplying the size difference of the comps (ratio) by adding the folowing expression to the null in the pre-comp:
mainPos = comp("mainComp").layer("Null 1").transform.position;
compRatio = thisComp.width / comp("mainComp").width;
mainPos * compRatio;