I'm not sure, but I think that deduplication ratio for your incremental backups is about 3 (blue to red ratio on the chart). Maybe incremental backups (with block change tracking) is a must, because of size of your database. But if there is a chance to do a full backup in a working day backup window, maybe You should try full backups for couple days without incrementals.
BTW. What is the reason that full backups have so big diffrence in size before deduplication? Maybe your DB really has huge changes week to week?