My hunch about your problem is how mongodb grabs space when it needs to be expanded. I think, but not sure of the exact algorithm, that it will take up twice as much space as it just requested. So, for example, as soon as he needs to expand beyond 400, he will ask for an additional 800, giving you 1.2 concerts.
To find out how much disk space you are using, you can run this command:
rhc app show {appname} -gears quotap >