I am using the Publisher Beta since some months ago as I am trying to replace Quark Express and InDesign.
I have seen through the different versions released that there is a common problem when handling the size of the file created.
It seems as if the program does not clean the information that has been deleted so when you are working on a design the size file increases and increases.
When you use the "save as" command and sabe the file under a different name the size of the document decreases a lot.
I would bet there is some historic information that it is stored in the file and it is just deleted when saving to a new document.
I am using a Surface pro 4, running the latest update of Windows 10 and 8Gb RAM. The sises of the documents handled are around 1,2 Gb