At the beginning of the month, Valve rolled out this stable client update:

An update has been released for the Steam Client and will be automatically downloaded.

General
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- Reduced the amount of disk space necessary to decrypt a preloaded game
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Click to expand...

That note popped into my head while I began waiting for Arkham Knight to unpack, so I thought I'd poke around and see what Valve changed. I found that Steam now splits pre-loaded data depots (archives) into 1GB chunks that are deleted on-the-fly, which means you no longer need roughly twice the amount of free space available when pre-loading a game (pre-load size + unpacked installation size), just the latter (the "Disk space required" figure reported when installing something) plus somewhere around a gig.

I'm sure SSD users in particular will be relieved, especially after GTAV.

Another wonderful piece of news! Thank you, Saoirse Ronan, for ensuring JaseC's existence on this planet!

Wow, I didn't even know this was a thing. That's pretty crazy.

This is great news - installing Witcher 3 on my SSD meant clearing 50GB of space first.