For very large apps, such a GPS apps, the app store will decline updating an app if you don't have enough free space.
How much free space is needed?
For instance, I have a GPS navigation app that is currently using 1.8GB on my iPad. The update is 1.43GB. I have 1.9GB free.
The update would fit into the free space, but it still doesn't allow it. Perhaps it downloads it, then installs it before deleting the old one, meaning you'd need twice as much space in order to install it. It might even require more space if it then copies user data from the old app into the new one before deleting the old one and the installation file.
Having as much free space as the app takes isn't enough - so how much is required? Is 2*update enough, or will I need more free space?