Ah, but a machine can work better or worse, and what is better or worse is often subjective, as there are various aspects of the workings of the machine regarding which you have to make trade-offs during the process of design. A notable example is the design of tanks: the bigger the cannon, the greater the weight, and the slower it gets as a result. You can try to get a better engine to compensate for the increased weight but a stronger engine and bigger guns lead to higher costs and complicate mass production.