Steam seems to have a bug currently that makes it start creating several gigabytes of files in ~/.local/share/Steam/config/htmlcache/DeferredBrowserMetrics.
These are very likely metrics recorded by Steam to be sent for telemetry purposes.
A solution is to delete all .pma files inside the directory, and then change the properties of the directory to make it read-only. When the directory is read-only, Steam won't be able to create new .pma files inside of it.