Are you embedding the fonts into your document?
Do you use live sync to cloud storage for your work in progress?
I was thinking that linked content in your document may be automatically downloaded if you are using Google Disk/MS OneDrive “Files on demand” feature (which I believe is enabled by default) or iCloud “Optimised storage”. That may account for the additional storage being reserved. Still a bit extreme, but if it isn’t versioning and also not tracked changes, that is the only other thing I can imagine causing it.
Expanded graphics may also inflate quite a bit, as others also commented, but I don’t believe it will be a factor of 1000x.
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@MirandaGrant have you checked to see what is taking up the space? You should be able to get a visual indicator in Apple menu > System Settings, click General in the sidebar, then click Storage on the right (wait for it to calculate) (from Free up storage space on Mac - Apple Support)
- If the biggest item is system data then some housekeeping might be required. This third party page has a lot of ads but some suggestions, What To Do if macOS System Data is Huge/Large - The Mac Observer
- If Documents or Apps increases massively when you open LibreOffice then it could be LibreOffice.