||Hmmm, while ArcMap documents can grow large because of the embedded graphics -- simply starting with a brand new mxd at a file size of 45 mb is not acceptable. Everytime you do lots of graphics changes on the layout, or insert and delete objects a lot in the mxd, the references to these objects are not removed automatically. That's what the "Save As" function does, as suggested.
There were issues I believe only during the Beta version of one of the earlier releases (9.1 or 9.2) that caused this, but if you didn't use beta, with this mxd, I would contact technical support for assitance and more suggestions to prevent that from happening.
You might try renaming your normal.mxt file and restarting ArcMap if starting any brand NEW mxd from scratch brings the file size automatically to 45mb. If it's an mxd file that's been around a while, it might have references to something that is missing or corrupt, such as a sample extension that you may have installed from ArcScripts or the developer samples.
Using the MXDoctor Utility might be a good place to try, too: http://support.esri.com/index.cfm?fa=knowledgebase.techarticles.articleShow&d=31424