Hi JAU,
The ATR was originally designed with go around buttons on both sides of both power levers, thereby having a small gab between the levers. However they realized that this was not the best solution as pilots fingers sometimes got stuck between the levers and they decided to fill out this gab and to have 1 go around button on each lever.. The cheepest solution was to make one lever bigger instead of two and that's why the left one is bigger today.
Hope that helps ??
Smiert spionom