Who changed the size of the transaction log?
Most likely you did. Most cases database files, including transaction log, are configured for auto-growth. Auth-growth enables the file to increase its size as needed, any time is needed. The trigger for growth can be any activity that requires log space. The activity that triggered the growth is most cases ordinary, normal activity. The true problem is why is the log growing. Most times is simply lack of log truncation, see Factors That Can Delay Log Truncation. Other time it can be a single event that generates a lot of log and requires growth. But most often is just bad maintenance (lack of truncation).

To shrink the log now, see How to Shrink the SQL Server log.

If the log was explicitly increased in size, the event is likely captured in the default administrative trace.