1)Save all data or just data that is not default?
Saving all data has the benefit of not requiring conversions (for example if the defaults ever changed) but at the cost of space
2) Write object desc def or just write all the data in the object desc
Saving all the data in the object desc creates a lot of waste (duplicative data), but it means that there is less synchronization if the object data ever changes.
For example:
versus
After all, you're using XML which is about the least space-efficient file format in existence. You've already made that decision, so there's no point in trying to save a few bytes now.