Someone took an ad-hoc full backup?
You can check this by restoring the header of the backups and look at (the sequence of) the LSN numbers.
Do you use compression on the backup? That could explain some differences in the size of the files.
As the database size is very large (around 2.5 TB) and according to hardware resource limitation, It's not possible to restore database to verify backup accuracy.
You should get some place where you can verify your backup by using a complete restore. Without a proper restore you will never know for sure if your backup is valid.
** Don't mistake the ‘stupidity of the crowd’ for the ‘wisdom of the group’! **