Here's my theory:
As you decreased the distance between front and back, and the front
galss bowed out, the volume of the tank would increase and the water
depth would lower, decreasing the water pressure on the glass.

The relationship is between depth and pressure. If you built a tank that was a
mile long and a mile wide, but six inches deep, you would need glass thick
enough to support the water pressure associated with a six-inch depth.

One has to remember that, assuming the tank is level, there is equal pressure
on all the vertical structures containing the water, not just the front glass.
As the tank expands in length or width, the amount of
vertical structure to contain the water increases correspondingly, and
continues to support its share of the pressure.