It is also an easy thing to check. Glue on some balsa and make the horizontal size something around 25%. Keep the aspect ration similar to the wing. If it is an area effects then being scientific about it trim off the additional area until the nose down effect is noticed again.

The basic thing the larger tail will do is to keep the airplane level long enough to see if the rudder can indeed yaw the airplane. Bigger motors don't let you have the fun of finding out why things happen.

As someone else suggested also check with different amounts of rudder input for both the big and small horizontal tails.

Then try all the above with properly faired canopy and nose areas.

This is an excellent opportunity to do some basic good aerodynamic research. Since you are looking for something causing a nose down pitch with rudder deflection it will be easy to determine if it is there or not.

I would like to encourage you to work on the problem and report the results. It is rare to have something this interesting to investigate that doesn't cost an arm and a leg.