10-15mb per day does seem high for a new OmniFocus user (unless you're attaching lots of files or something).
Would you mind emailing us?
omnifocus@omnigroup.com Please include your sync server user name, and let our Support Ninjas know I asked you to write in.
I don't know exactly how much data we can extract from our server logs, but we'll see what we can figure out for you. We can also take a look and see if your database is getting unnecessarily large and help you slim it back down.