Wow. 20 MB is very, very large.
Given that the iPhone only has 64 MB of memory to run the OS, any background applications (the phone, the ipod), and OmniFocus, that definitely explains why you're having trouble getting your database to load on the phone.
If the steps I mentioned in my post to dutch5050 didn't shrink it enough, contact the
support ninjas via email and we can help.