I'd recommend you to stay with the backup job followed by backup copy job with GFS scheme
as it corresponds to the best practices.
GFS restore points always will be the same size as .vbk file.
The backup copy job creates copy of .vbk on the particular day according to GFS scheme you've set up.
If you're using dedupe appliance, then you can switch backup method to the Forward incremental
, also known as an Active full method by selecting check box - Read the entire restore point from source...
By the way there is a tool that should help you to calculate approximate backup size - Restore point simulator