Unfortunately there is no central location to view what contributes to file size.
In our experience however, it is most often one of the following...
1. Layouts
2. External content, e.g. DWGs, PDFs or JPEGs
3. Sections or Elevations, if set to automatic update they will also slow your file down;
4. In case of teamwork files, the more users are joined the bigger the file gets... only resharing will help.
Btw. If you are concerned about speed of your file - there are also other factors such as hardware and 3D modelling techniques to consider - let us know how you go.