In many applications, the document starts real small and then gets bigger quickly, for example when storing comments or dialog lines in a list.
In the past we sometimes told people to manually add dummy fields to ensure the document will not be migrated too many times, but that is messy.
Even if you can set the padding factor manually on a collection (not available yet), it will not be a precise setting and can still lead to much fragmentation.
The cleanest way to solve is to be able to specify a default minimum document size, easy to understand and use.