The issue of control heaviness is more complex than what has been stated here, as heavy controls can force a pilot to make large control inputs than would otherwise be required and then a corrosponding larger correction. This is known as "higher gain", and is a known factor in leading to PIO, or, more correctly, aircraft-pilot coupling (APC). Either high or low control forces can lead to APC. One of the main reasons the Wright Bros were successful is that they made their craft very unstable, which, as the time, was more controllable than the too stable aircraft designed by others. Those others were not controllable, although the Wright aircraft were virtually also not controllable, they are more controllable than too stable designs.
In the case of the close formation flying, the tight tracking task of flying in that formation will push a low gain, anticipatory type pilot towards the higher gain/reactionary mode. The constant control pressure was a way of forcing the gain down during the required tight tracking maneuvers they fly.