the project varies in size as we add or remove items (due to deletions, varioations etc) and currently stands at 4807 tasks, of which about 60% are complete (actual start/finish dates added) and maybe 2-300 tasks are in progress.

I tried the "save as" immediately after loading, no difference in file size. Currently I am at 53 040 kb (just over 53 meg), the strange saves double this to 106 or so meg.
The programme undergoes at least 3 or 4 revisions every day, due to amended works, updating progress, adding/ removing works, and reprogramming to allow for revosed labour etc.