What actually makes a SQL log file grow?

I am trying to get a better understanding of what makes a SQL log file grow. For most of our clients they do not implement a point in time backup procedure so the recover model is set to Simple. Their databases tend to be fairly small(less than 10GB). I routinely perform upgrades to their ERP solution and during the upgrade process the log file may grow to 8GB due to all the insert, update and delete commands. Now sometimes a client does not upgrade at each revision so I need to perform a multi-hop upgrade to get them from the version they are on to the most current version. Now I just conducted an experiment. I just performed a four hop upgrade. At each upgrade I saw the log file grow to about 8GB. I then shrank the log file before performing the next upgrade. I then performed the complete upgrade without shrinking at each hop. I was surprise to see that the log file was about the same 8GB. Why didn't the log file grow to 32GB? Is SQL reusing some of the space in the log file?