3 cylinder engines are very unbalanced. Wouldn't making the middle cylinder bigger improve the balance of the engine?
No, because you don't change the fundamental harmonic imbalance, just change it's location in the cycle.
A large center cylinder would/could solve the rotating mass imbalance of a 3 cylinder. You trade it instead for a huge airflow, combustion, and power output imbalance that would be incredibly difficult to engineer out for a good running engine. The center cylinder will develop much more torque, take longer to burn fuel, get hotter, and result in a lopey torque application that bodes badly for bearing and crankshaft life.