I made it this way. On the anchor property of your size-modifiyng layer add the expression
temp = thisLayer.sourceRectAtTime(time,false).height/2;
[value[0],temp]
and on the size position (Rectangle 1>Rectangle path 1> Size) addd the expression
temp = (thisComp.layer("Shape Laye 2").transform.position[1]*-1 + height)/2;
[value[0], temp]
replace “height” with the starting coordinates of your layers, or align the size-modifiyng layer to the bottom of the composition. I added an image for easier understanding.

Andrei