Snapshot “grows” due to data changes, not due to data additions. After adding 20G of data snapshot size will remain the same (almost because of metadata overhead), but after changing 20G of data size of active file system will not change, but snapshot will appear to “grow” to 20G.
I suggest getting file modification time report to see, how many files are changed. You mentioned in another post that most files are office documents. In this case every changed file almost sure implies completely new version with old one possibly frozen in snapshot.
Get several consecutive snapshots; compare file modification times to see whether files were changed between snapshots. I suspect sum of changed file sizes will be close to snapshot size.
You could also enable audit to get exact report who is accessing files.