Please have a look into your OmniFocus backup folder now. It typically is at ‘~/Documents/OmniFocus Backups’ unless you configured it differently in OmniFocus’ preferences.
How big is it? 500MB? 2GB or even 16GB? Mine was 2GB yesterday since I didn’t clean up old backups since half a year.
There is of course a more efficient way that does not even require you to remember this maintenance task at all: Using Hazel and the rule described by Christopher Mackay will do the job for you.