There are a whole range of ways you can save the presentation file incorrectly The most common of these are
Back saving “ saving for use in a previous version
Leaving Fast Saves on
Not saving often enough
Not saving a backup
All of these problems can cause you grief . The first two will give you large files are likely to corrupt easily. The last two will cause you to lose data. Remember, there are two kinds of computer users: those who have lost data and those who will.
First, turn off Fast Saves (Tools ’ Options ’ Save tab). Next, don't save the presentation for use in a previous version unless you need to open the file in PowerPoint 95. Every version of PowerPoint since 97 uses the same file format. You don't need to back save unless you are sharing data with a very old version. (By the way, if you are sharing your presentation with a computer that only has PowerPoint 95 on it, you will run into many more problems than the file size .)
Save the file on the hard drive frequently. Save it with a new name at least once for every hour of work you have into the file. If you can't remember to save the file with a new name, get the Sequential Save add-in and use it.