Mac pet peeve

The Software Update application on my Mac tells me that I need to upgrade the operation system:

The 10.5.6 Update is recommended for all users running Mac OS X Leopard and includes general operating system fixes that enhance the stability, compatibility and security of your Mac.

The dialog claims that it's a 675 MB file.

If I actually push the Install button, I get the following message:

4.9 GB is required to download and install the updates. Extra space is required for the optimization process. To free up space, remove files from your startup disk, then try again.

How on earth do they get from 675 MB to 4.9 GB? That must be some kind of optimization that requires nearly an order of magnitude of space than the data itself. Only Apple makes it possible.

I remember when a perfectly serviceable multitasking operating system with a graphical user interface fit into less than 256 KB of memory.