You're mixing up some terms. "Log truncation", for most of us, means that data in the transaction log file has been deleted because whatever transaction caused it has been completed and committed AND a log file backup has occurred. That process does not decrease the size of the log file.

What you're talking about is the "shrinking" of the file-space used and that doesn't happen automatically just because data has been deleted from the log file.

If this is a regular problem (and, for restore purposes, I do agree that the log file is much bigger than it should be), consider temporarily switching the database to the "BULK LOGGED" Recovery Model during the index rebuilds of large tables so that you can take advantage of "Minimal Logging". There is a caveat to that, though. You need to read up on the effect that minimally logged operations can have on Point-in-Time restores.