OK in case it’s of use to anyone in a similar situation -
I analysed my model using Audit3dmFile
It turns out that spheres add significantly to the file size. I added a lot of spheres yesterday!
Each of these is adding 250 - 350kb to the model -
I see I’m not the first person to encounter issues with spheres creating a massive file size.
I’m going to try some of the suggestions under this thread to get my file back under control - Why is my file size so large?