As installed the first floor and second floor has 16x20 returns. One is in the first floor ceiling and one is above the door at the air handler. The first floor return is reduced to a 12" x 12" box installed at the floor of the air handler (why reduced) while the second floor above the door stays the same.
The return intake is on the top of the air handler just 2' away from the grill above the door if not less. The small return at the floor is 5' or so. Air will be drawn from the path of least resistance which would be above the door. Granted air will still be drawn by the lower but in a much decreased volume.
One would think that the first floor return would be slightly larger then the second floor being the first floor has more square feet and termination point at the floor.