Why is the First Transaction Log backup of the day huge
That depends on what is happening in the night. 


If you have additional maintenance jobs (index rebuilds, index reorganizations, statistics updates) then you possibly have a lot of changes occurring.  
If you are also importing large amounts of data, then that is another possible source of changes.  
Deleting data is also a modification and will be logged. 


A database that is truly dormant will normally not have huge transaction log backups.

You may also want to consider either taking a second full backup and a transaction log backup after the nightly maintenance, and use that backup as your base for the day's log backups solely to avoid the massive post-maintenance log file, or to just postpone the full backup and subsequent transaction log backups till after the maintenance.