Why a small Steam update can need a lot of free disk space

A Steam update can finish downloading while the client is still applying the patch. The download size tells you how much data Steam transfers. Applying the update can require much more space because Steam may be building replacement files using data already on your drive.

What Steam does with the download

Many PC games group assets such as textures and sounds into large pack files. SteamPipe, Valve’s content delivery system, can reuse chunks of the old file and download the chunks it needs. For a pack file that changes, SteamPipe builds the replacement alongside the existing file. After the new files are ready, it removes the old ones and moves the replacements into place. Valve illustrates this with a 25 GB pack file that needs another 25 GB of local space for its replacement even when only a tiny part changes. That figure describes a single example in Valve’s documentation. Steamworks documentation on SteamPipe

Measure your next update

1. Queue one game update and pause other Steam downloads. Note the game, update version if available, and free space on the drive where the game is installed. On Windows, you can check the drive under This PC in File Explorer. Take the first reading before starting the update.
2. Start the update and note the download size shown in Steam Downloads. Refresh the drive’s free-space reading every few seconds while Steam downloads and applies the update. Write down the lowest reading you see.
3. When the update finishes, record the free space again. Subtract the lowest reading from the starting reading to estimate the largest drop you observed. Subtract the final reading from the starting reading to estimate how much extra drive space is still in use after the update.

Sampling can miss the lowest point between readings, and background writes can change the drive’s free space. Treat the result as an estimate for that run. Steam Support shows where to find Downloads and manage its queue. Steam Support’s downloads and updates guide

If the update runs out of space

Check the drive that holds the game first. If you have another drive with an existing Steam library and more room, open the game’s Properties, select Installed Files, then choose Move Install Folder. Steam can move an existing installation between its libraries without downloading the game again. Retry the update after the move completes. Steam Support’s instructions for moving game installations