Why does Snow Leopard free so much hard disk space?
John Siracusa’s review of Mac OS X 10.6 lists several space freeing features (on page 3). The biggest contributors are “install on demand components”, dropping support for PowerPC processors, and “HFS+ compression”.


The Rosetta PowerPC binary translator software and the bulk of the printer drivers and are not installed by default.

They can be installed from DVD or downloaded when they are needed.
The system-installed programs have dropped support for PowerPC processors.  

At least one architecture was eliminated from every program on the system (32-bit PPC). Some of them dropped two architectures (32- and 64-bit PPC). This means that most Snow Leopard executables only store code for half the number of ISAs as the equivalent Leopard binaries. The different ISAs take different amounts of binary code to represent the same high-level program, so this does not always mean a 50% space savings for each binary. Nevertheless, “50% byte savings for each executable” is a fairly accurate estimate.  
Many of the Snow Leopard files are also transparently compressed.  

Limited access to HFS+ compression is provided in through the 
ditto(1) and 
afscexpand(1) commands. Some third part tools provide ways to identify or find such compressed files, but this is generally not a “user level” feature.


The review also mentions the change to powers-of-ten for reporting large byte sizes (1 GB = 1,000,000,000 (10^9) bytes instead of 1,073,741,824 (2^30) bytes) in the “A gigabyte by any other name” sidebar.