If you want to nitpick...
Full backups log the checkpoint start and end, the changes to the database boot page and a couple other things.
To grow a log that starts with default settings (1MB, 10%) to 30MB by full backups alone requires around 2300 full backups. At one backup a day (about the most I've ever seen for model backups) that's close on 6 and a half years of backups. At one backup a week (the most frequent that I would suggest), that's around 44 years of backups.
I stand by my original comment. If you are seeing model (visibly) grow, something is using the database - creating objects, modifying data, etc.
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SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
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