I had a similar problem when I was doing a lot of editing and saving of a project file and it increased dramatically in size. Please see below the rationale and fix.
There are various reasons Project files get large. One is simply that the file contains several thousand tasks and has a lot of data. However, Project files can also become bloated due to heavy editing and continued use of the “Save” feature. Using “Save” appends new data and does not overwrite the existing file, so the file bloats. You can use “Save As” to the same file name. You can also have Project do some file “housekeeping” by opening and then immediately closing a file.
Because of the way Project stores files, file fragmentation may be the basic cause. To make sure this isn’t the problem: open your .mpp file then select File/Save immediately before doing any other action. Only if you save immediately after opening does Project defragment its .mpp file However, be advised that using Save As with linked files can actually create corruption/bloat due to multiple link paths being created IF the file is not saved back to its original name and path.