NZ Rail Maps: Two different ways to cover a large area in Gimp [2]

Following on from my previous post, in the case of the Dunedin-Wingatui map mosaic Gimp project, I chose to extend the size of the map canvas from 7×7 to 11×7 and bring the tiles for the Wingatui station alongside those of Green Island-Abbotsford. The reason for this was to be able to bring some smaller scale aerial images of the Walton Park branch into the project and easily overlaying them over the large area that stretches from Green Island to Wingatui.
This means this project now has a significantly larger canvas size than before and after adding two new Retrolens aerial photos covering the area in question and saving into a new file, the size of this file has naturally increased as would be expected with adding new content. In this case it went up by 5 GiB. I am not so sure how much of this is increased canvas size and how much is new content, as the two new retrolens images cover a significant area – a corridor length of more than 3 km. In order to test this further I am going to increase canvas size again just to lay the Mosgiel LDS tiles (which are already in the project) linearly alongside Green Island-Wingatui, and then save again and see how much increase if any in file size there is.
It is of course also opportune to ask about the merits of very large projects combining multiple locations into one file compared with single location files. One of the challenges with Gimp is the save time for a 30 GB project can be more than an hour. This, along with the time it takes for Gimp to run a unified transform over a large layer, is the key reason I have two computers working on the maps, as whilst this delay is occurring, I can use the other computer to work on something else. Having one of the computers almost solely for the purpose of running Gimp is not such a far fetched concept after all. 
Having a lot of stuff in one file is meritous mainly for efficiency, both in reduced number of disk files, and in combining and overlapping aerial images together. However, the overlapping is really only relevant where you have two or more stations that are fairly close together like in the main centres. For some areas where stations are further apart that I have combined them in one file, it is mostly for efficiency and organisation purposes.