I would try manual solution below or paid software Project Repair Toolbox
https://www.repairtoolbox.com/projectrepair.html in case first variant doesn't work or you have severe corruption of .mpp file.
Re-create a whole new file from the corrupted/bloated file:
1. Open a new blank project file
2. Go to Insert/Project and insert the problem file.
3. Select the first task which will be the insertion point summary line.
4. Go to Project/Task Information/Advanced tab.
5. Uncheck the "Link to Project" button and hit "OK".
6. If the file isn't already expanded, expand it.
7. Select all tasks below the top level summary line.
8. Out dent the selected tasks.
9. Delete the first task which previously was a summary line.
10. Save the resulting file.