Animating query based on mask.

The easiest approach would be to use the Window/Create Nulls From Paths/Nulls Follow Points script, then delete all but one null on the Right Edge of the animated mask and one null on the left edge. You can get rid of the control Null. Then you use the X position of the nulls to create a pair of ease interpolation methods that use the width of the arrows as the tMin and tMax values and the X Position as t, then set the starting and ending value for scale on each arrow for value1 and value2.

Something like this would work:

t = thisComp.layer("Adjustment Layer 1: Mask 1 [1.0]").position[0];
t2 = thisComp.layer("Adjustment Layer 1: Mask 1 [1.1]").position[0];
tMin = position[0] + sourceRectAtTime().width;
tMax = tMin - sourceRectAtTime().width * 2;
v = ease(t, tMin, tMax, 100, 200);
v2 = ease(t2, tMax, tMin, 200, 100);
if (t < tMin)
	s = v;
else
	s = v2;
[s, s]

If you choose Nulls Follow Points and apply this to all shape layer Scale properties you should end up with arrows that scale from 100 to 200% as the shape layer mask passes. 

That should get you pretty close. You might have to modify the expression a bit to get the scale to accurately follow the leading and trailing edge of the mask. I don't have time to play with it to perfect it right now. 

Probably something with sampleImage() or sourceRectAtTime() or possibly simple distance calculations if the mask is a regular shape. A screenshot would be more useful since I can't open your project here at home.

Mylenium