The increase in hard drive capacity is impressive: in the last 15 years, storage density has gone up 10,000x!
The storage industry is still finding ways to keep increasing storage densities. The latest technology is called Perpendicular Magnetic Recording (PMR), which moves magnetization of particles from a horizontal to a vertical alignment, so more bits can fit the same space.
If you take 1991's maximum capacity of 130 MB and compare it to today's 750 GB per hard drive, capacities have increased by a factor of 5,700 in the last 15 years. If you compare the per-platter capacity, the difference is even larger.