Your bandwidth usage is calculated by number of people using your app multiplied by the size of your app. Each content update will be slightly smaller than the full size of the application. When you publish a content update, each user's mobile device will only download assets that are missing, or assets that have changed in your update.
Videos or any other assets hosted outside of App Press (YouTube videos, for example) are not included in this calculation.
Here's the monthly bandwidth for each plan, with overage pricing by plan level: