The reason one gains strength without increasing muscle mass is many fold. First, there are the intramuscular forms of improving strength:
- Increased neuromuscular efficiency is the main reason. Basically, your central nervous system is able to activate a greater number of motor units, and therefore muscle fibers, to complete the task at hand.
- Rate coding is also very important. Essentially, a higher level of rate coding means that motor units have increased their firing rates, which leads to increased force generation.
- Motor unit synchronization can also help. I think the name of this form of adaptation is pretty self-explanatory.
- Increased rate of force generation is yet another reason. More motor units can be activated in less time. This creates a greater acceleration of the weight being moved.
Intermuscular improvements are also important. The basic idea is that your body, and more specifically your central nervous system, can coordinate movements among separate muscles in such a way that the desired movement is performed in the most efficient manner possible.