I agree a 'BallotBox' that is more than one meg is probably unrealistic. In this case it was >28000 ballots which would take 19 days of continuous 24 hour a day voting assuming 1 minute per vote.
The problem we have though is that the current "Generate ballot test" which is used to verify the ballot layout/recording logic by counties can generate more than this. We would need to modify this logic to reduce the number of ballots that are generated and also decide what to do when the 'ballotbox' get > 1M. For now it may just be easier to allow 'ballotboxes' to be upto 4M.