I stopped using steam because it seems to expect a broadband internet connection. It will often (sometimes without prompting) overwrite and re-download gigabytes of data
Recently it seems to be better about respecting download settings. The download size also seems to be an upper limit, for example Team Fortress 2 pretty much always showed up as a 10gb update download, but spent less than 5 minutes actually downloading files. Not sure if that estimation has been fixed recently, but it used to be the case.