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. If you do not have external links in your file, (i.e. resource pool, master with subprojects, etc.), then a good practice is to always use Save As instead of a straight Save with your file after editing. Or you can try to use
mpp repair, very nice paid service. This helps keep editing clutter from being appended to the file. However, be advised that using Save As with linked files can actually create corruption/bloat due to multiple link paths...