* Subgiant Stars: These are stars in the process of evolving from main sequence stars (like our Sun) to giant stars. They are still fusing hydrogen in their cores but are starting to expand and cool. They are larger than main sequence stars but not as large as giants.

* Neutron Stars: These are the incredibly dense remnants of massive stars that have exploded as supernovae. They are only a few miles in diameter, making them extremely compact.

In short: While a neutron star is incredibly dense, it's incredibly small. A subgiant star, on the other hand, is much larger in terms of physical size.