Dynamic speakers/drivers are used in a variety of devices, often requiring that they be altered to fit the application. Large PA speakers, for instance, generally have large diaphragms that can produce loud, low frequencies. As you would imagine, the bigger the speaker, the bigger the sound waves that can be created. This is easily demonstrated by sub speakers, which are commonly larger than other speakers and are specifically designed to produce lower frequencies.
When speakers need to be smaller, as in the case of headphones, the power and frequency response may need to be altered. The power is not altered too much since the driver is going to be closer to the ear and so does not need to put out as much energy. However, the response may need to be engineered to get the desired lower frequencies from the smaller-sized speakers. Engineers use different techniques to overcome the size limitations and achieve this lower frequency reproduction.