You have to understand what is happening when you move the lever. With Shimano shifters, when you move the brake lever sideways on either side you are pulling the shift cable against the spring tension in the derailleur to move it a specified distance. If you don't move that brake lever that distance, no shift will happen. This amount of lever travel cannot be adjusted. In the other direction, the smaller levers behind the brake levers require only a small amount of travel, all you are doing is releasing a single gear change, but you are aided by the spring in the derailleur.