Also for general knowledge: very long projects or projects with large numbers of assignments per task that then also have several baselines saved can get big. Combinations there of all can get large as the number of assignment timeslices that must be stored goes up.

Clearing unused baselines can decrease size by big amounts.

I have found this reduces the file size in some cases up to 50%.

Method 1
Save your file as a Microsoft Project Database file (.mpd), and then open and resave it as a Microsoft Project file (.mpp). To do this, follow these steps:
1. Start Microsoft Project, and then open your file.
2. Click OK in response to the error message described in the "Symptoms" section of this article.
3. On the File menu, click Save As.
4. In the Save As dialog box, specify a file name and location in which to save your file.
5. In the Save as type box, click Project Database (*.mpd), and then click Save.
6. In the Save To Database dialog box, click Save.
7. Click Close on the File menu.
8. Open the .mpd file that you saved in step 5.
9. On the File menu, click Save As.
10. In the Save As dialog box, specify a file name and location in which to save your file.
11. In the Save as type box, click Project (*.mpp), and then click Save.