Actually, at the first engagement the Executor participated in, a miscalculation upon exiting hyperspace, caused three Imperial-class destroyers to crash into the Ex. These were all destroyed, while the Executor's shields saved it from anything bigger than a few weeks in repairdock. The impact of a fighter-sized projectile, like the A-wing, would cause little damage. But, since the shields (both primary and auxillery) were down, the impact was much more severe and led to its downfall as seen.